sanitize content of README file

Current README content is way too esoteric for someone looking at GIT
for the first time. Instead it should provide a quick summary of what
GIT is with a few pointers to other resources.

The bulk of the previous README content is moved to
Documentation/core-intro.txt.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 590 insertions, 0 deletions

diff --git a/Documentation/core-intro.txt b/Documentation/core-intro.txt
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+////////////////////////////////////////////////////////////////
+
+	GIT - the stupid content tracker
+
+////////////////////////////////////////////////////////////////
+
+"git" can mean anything, depending on your mood.
+
+ - random three-letter combination that is pronounceable, and not
+   actually used by any common UNIX command.  The fact that it is a
+   mispronunciation of "get" may or may not be relevant.
+ - stupid. contemptible and despicable. simple. Take your pick from the
+   dictionary of slang.
+ - "global information tracker": you're in a good mood, and it actually
+   works for you. Angels sing, and a light suddenly fills the room.
+ - "goddamn idiotic truckload of sh*t": when it breaks
+
+This is a (not so) stupid but extremely fast directory content manager.
+It doesn't do a whole lot at its core, but what it 'does' do is track
+directory contents efficiently.
+
+There are two object abstractions: the "object database", and the
+"current directory cache" aka "index".
+
+The Object Database
+~~~~~~~~~~~~~~~~~~~
+The object database is literally just a content-addressable collection
+of objects.  All objects are named by their content, which is
+approximated by the SHA1 hash of the object itself.  Objects may refer
+to other objects (by referencing their SHA1 hash), and so you can
+build up a hierarchy of objects.
+
+All objects have a statically determined "type" aka "tag", which is
+determined at object creation time, and which identifies the format of
+the object (i.e. how it is used, and how it can refer to other
+objects).  There are currently four different object types: "blob",
+"tree", "commit" and "tag".
+
+A "blob" object cannot refer to any other object, and is, like the type
+implies, a pure storage object containing some user data.  It is used to
+actually store the file data, i.e. a blob object is associated with some
+particular version of some file.
+
+A "tree" object is an object that ties one or more "blob" objects into a
+directory structure. In addition, a tree object can refer to other tree
+objects, thus creating a directory hierarchy.
+
+A "commit" object ties such directory hierarchies together into
+a DAG of revisions - each "commit" is associated with exactly one tree
+(the directory hierarchy at the time of the commit). In addition, a
+"commit" refers to one or more "parent" commit objects that describe the
+history of how we arrived at that directory hierarchy.
+
+As a special case, a commit object with no parents is called the "root"
+object, and is the point of an initial project commit.  Each project
+must have at least one root, and while you can tie several different
+root objects together into one project by creating a commit object which
+has two or more separate roots as its ultimate parents, that's probably
+just going to confuse people.  So aim for the notion of "one root object
+per project", even if git itself does not enforce that.
+
+A "tag" object symbolically identifies and can be used to sign other
+objects. It contains the identifier and type of another object, a
+symbolic name (of course!) and, optionally, a signature.
+
+Regardless of object type, all objects share the following
+characteristics: they are all deflated with zlib, and have a header
+that not only specifies their type, but also provides size information
+about the data in the object.  It's worth noting that the SHA1 hash
+that is used to name the object is the hash of the original data
+plus this header, so `sha1sum` 'file' does not match the object name
+for 'file'.
+(Historical note: in the dawn of the age of git the hash
+was the sha1 of the 'compressed' object.)
+
+As a result, the general consistency of an object can always be tested
+independently of the contents or the type of the object: all objects can
+be validated by verifying that (a) their hashes match the content of the
+file and (b) the object successfully inflates to a stream of bytes that
+forms a sequence of <ascii type without space> + <space> + <ascii decimal
+size> + <byte\0> + <binary object data>.
+
+The structured objects can further have their structure and
+connectivity to other objects verified. This is generally done with
+the `git-fsck-objects` program, which generates a full dependency graph
+of all objects, and verifies their internal consistency (in addition
+to just verifying their superficial consistency through the hash).
+
+The object types in some more detail:
+
+Blob Object
+~~~~~~~~~~~
+A "blob" object is nothing but a binary blob of data, and doesn't
+refer to anything else.  There is no signature or any other
+verification of the data, so while the object is consistent (it 'is'
+indexed by its sha1 hash, so the data itself is certainly correct), it
+has absolutely no other attributes.  No name associations, no
+permissions.  It is purely a blob of data (i.e. normally "file
+contents").
+
+In particular, since the blob is entirely defined by its data, if two
+files in a directory tree (or in multiple different versions of the
+repository) have the same contents, they will share the same blob
+object. The object is totally independent of its location in the
+directory tree, and renaming a file does not change the object that
+file is associated with in any way.
+
+A blob is typically created when gitlink:git-update-index[1]
+is run, and its data can be accessed by gitlink:git-cat-file[1].
+
+Tree Object
+~~~~~~~~~~~
+The next hierarchical object type is the "tree" object.  A tree object
+is a list of mode/name/blob data, sorted by name.  Alternatively, the
+mode data may specify a directory mode, in which case instead of
+naming a blob, that name is associated with another TREE object.
+
+Like the "blob" object, a tree object is uniquely determined by the
+set contents, and so two separate but identical trees will always
+share the exact same object. This is true at all levels, i.e. it's
+true for a "leaf" tree (which does not refer to any other trees, only
+blobs) as well as for a whole subdirectory.
+
+For that reason a "tree" object is just a pure data abstraction: it
+has no history, no signatures, no verification of validity, except
+that since the contents are again protected by the hash itself, we can
+trust that the tree is immutable and its contents never change.
+
+So you can trust the contents of a tree to be valid, the same way you
+can trust the contents of a blob, but you don't know where those
+contents 'came' from.
+
+Side note on trees: since a "tree" object is a sorted list of
+"filename+content", you can create a diff between two trees without
+actually having to unpack two trees.  Just ignore all common parts,
+and your diff will look right.  In other words, you can effectively
+(and efficiently) tell the difference between any two random trees by
+O(n) where "n" is the size of the difference, rather than the size of
+the tree.
+
+Side note 2 on trees: since the name of a "blob" depends entirely and
+exclusively on its contents (i.e. there are no names or permissions
+involved), you can see trivial renames or permission changes by
+noticing that the blob stayed the same.  However, renames with data
+changes need a smarter "diff" implementation.
+
+A tree is created with gitlink:git-write-tree[1] and
+its data can be accessed by gitlink:git-ls-tree[1].
+Two trees can be compared with gitlink:git-diff-tree[1].
+
+Commit Object
+~~~~~~~~~~~~~
+The "commit" object is an object that introduces the notion of
+history into the picture.  In contrast to the other objects, it
+doesn't just describe the physical state of a tree, it describes how
+we got there, and why.
+
+A "commit" is defined by the tree-object that it results in, the
+parent commits (zero, one or more) that led up to that point, and a
+comment on what happened.  Again, a commit is not trusted per se:
+the contents are well-defined and "safe" due to the cryptographically
+strong signatures at all levels, but there is no reason to believe
+that the tree is "good" or that the merge information makes sense.
+The parents do not have to actually have any relationship with the
+result, for example.
+
+Note on commits: unlike real SCM's, commits do not contain
+rename information or file mode change information.  All of that is
+implicit in the trees involved (the result tree, and the result trees
+of the parents), and describing that makes no sense in this idiotic
+file manager.
+
+A commit is created with gitlink:git-commit-tree[1] and
+its data can be accessed by gitlink:git-cat-file[1].
+
+Trust
+~~~~~
+An aside on the notion of "trust". Trust is really outside the scope
+of "git", but it's worth noting a few things.  First off, since
+everything is hashed with SHA1, you 'can' trust that an object is
+intact and has not been messed with by external sources.  So the name
+of an object uniquely identifies a known state - just not a state that
+you may want to trust.
+
+Furthermore, since the SHA1 signature of a commit refers to the
+SHA1 signatures of the tree it is associated with and the signatures
+of the parent, a single named commit specifies uniquely a whole set
+of history, with full contents.  You can't later fake any step of the
+way once you have the name of a commit.
+
+So to introduce some real trust in the system, the only thing you need
+to do is to digitally sign just 'one' special note, which includes the
+name of a top-level commit.  Your digital signature shows others
+that you trust that commit, and the immutability of the history of
+commits tells others that they can trust the whole history.
+
+In other words, you can easily validate a whole archive by just
+sending out a single email that tells the people the name (SHA1 hash)
+of the top commit, and digitally sign that email using something
+like GPG/PGP.
+
+To assist in this, git also provides the tag object...
+
+Tag Object
+~~~~~~~~~~
+Git provides the "tag" object to simplify creating, managing and
+exchanging symbolic and signed tokens.  The "tag" object at its
+simplest simply symbolically identifies another object by containing
+the sha1, type and symbolic name.
+
+However it can optionally contain additional signature information
+(which git doesn't care about as long as there's less than 8k of
+it). This can then be verified externally to git.
+
+Note that despite the tag features, "git" itself only handles content
+integrity; the trust framework (and signature provision and
+verification) has to come from outside.
+
+A tag is created with gitlink:git-mktag[1],
+its data can be accessed by gitlink:git-cat-file[1],
+and the signature can be verified by
+gitlink:git-verify-tag[1].
+
+
+The "index" aka "Current Directory Cache"
+-----------------------------------------
+The index is a simple binary file, which contains an efficient
+representation of a virtual directory content at some random time.  It
+does so by a simple array that associates a set of names, dates,
+permissions and content (aka "blob") objects together.  The cache is
+always kept ordered by name, and names are unique (with a few very
+specific rules) at any point in time, but the cache has no long-term
+meaning, and can be partially updated at any time.
+
+In particular, the index certainly does not need to be consistent with
+the current directory contents (in fact, most operations will depend on
+different ways to make the index 'not' be consistent with the directory
+hierarchy), but it has three very important attributes:
+
+'(a) it can re-generate the full state it caches (not just the
+directory structure: it contains pointers to the "blob" objects so
+that it can regenerate the data too)'
+
+As a special case, there is a clear and unambiguous one-way mapping
+from a current directory cache to a "tree object", which can be
+efficiently created from just the current directory cache without
+actually looking at any other data.  So a directory cache at any one
+time uniquely specifies one and only one "tree" object (but has
+additional data to make it easy to match up that tree object with what
+has happened in the directory)
+
+'(b) it has efficient methods for finding inconsistencies between that
+cached state ("tree object waiting to be instantiated") and the
+current state.'
+
+'(c) it can additionally efficiently represent information about merge
+conflicts between different tree objects, allowing each pathname to be
+associated with sufficient information about the trees involved that
+you can create a three-way merge between them.'
+
+Those are the three ONLY things that the directory cache does.  It's a
+cache, and the normal operation is to re-generate it completely from a
+known tree object, or update/compare it with a live tree that is being
+developed.  If you blow the directory cache away entirely, you generally
+haven't lost any information as long as you have the name of the tree
+that it described.
+
+At the same time, the index is at the same time also the
+staging area for creating new trees, and creating a new tree always
+involves a controlled modification of the index file.  In particular,
+the index file can have the representation of an intermediate tree that
+has not yet been instantiated.  So the index can be thought of as a
+write-back cache, which can contain dirty information that has not yet
+been written back to the backing store.
+
+
+
+The Workflow
+------------
+Generally, all "git" operations work on the index file. Some operations
+work *purely* on the index file (showing the current state of the
+index), but most operations move data to and from the index file. Either
+from the database or from the working directory. Thus there are four
+main combinations:
+
+1) working directory -> index
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You update the index with information from the working directory with
+the gitlink:git-update-index[1] command.  You
+generally update the index information by just specifying the filename
+you want to update, like so:
+
+	git-update-index filename
+
+but to avoid common mistakes with filename globbing etc, the command
+will not normally add totally new entries or remove old entries,
+i.e. it will normally just update existing cache entries.
+
+To tell git that yes, you really do realize that certain files no
+longer exist, or that new files should be added, you
+should use the `--remove` and `--add` flags respectively.
+
+NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
+necessarily be removed: if the files still exist in your directory
+structure, the index will be updated with their new status, not
+removed. The only thing `--remove` means is that update-cache will be
+considering a removed file to be a valid thing, and if the file really
+does not exist any more, it will update the index accordingly.
+
+As a special case, you can also do `git-update-index --refresh`, which
+will refresh the "stat" information of each index to match the current
+stat information. It will 'not' update the object status itself, and
+it will only update the fields that are used to quickly test whether
+an object still matches its old backing store object.
+
+2) index -> object database
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You write your current index file to a "tree" object with the program
+
+	git-write-tree
+
+that doesn't come with any options - it will just write out the
+current index into the set of tree objects that describe that state,
+and it will return the name of the resulting top-level tree. You can
+use that tree to re-generate the index at any time by going in the
+other direction:
+
+3) object database -> index
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You read a "tree" file from the object database, and use that to
+populate (and overwrite - don't do this if your index contains any
+unsaved state that you might want to restore later!) your current
+index.  Normal operation is just
+
+		git-read-tree <sha1 of tree>
+
+and your index file will now be equivalent to the tree that you saved
+earlier. However, that is only your 'index' file: your working
+directory contents have not been modified.
+
+4) index -> working directory
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You update your working directory from the index by "checking out"
+files. This is not a very common operation, since normally you'd just
+keep your files updated, and rather than write to your working
+directory, you'd tell the index files about the changes in your
+working directory (i.e. `git-update-index`).
+
+However, if you decide to jump to a new version, or check out somebody
+else's version, or just restore a previous tree, you'd populate your
+index file with read-tree, and then you need to check out the result
+with
+
+		git-checkout-index filename
+
+or, if you want to check out all of the index, use `-a`.
+
+NOTE! git-checkout-index normally refuses to overwrite old files, so
+if you have an old version of the tree already checked out, you will
+need to use the "-f" flag ('before' the "-a" flag or the filename) to
+'force' the checkout.
+
+
+Finally, there are a few odds and ends which are not purely moving
+from one representation to the other:
+
+5) Tying it all together
+~~~~~~~~~~~~~~~~~~~~~~~~
+To commit a tree you have instantiated with "git-write-tree", you'd
+create a "commit" object that refers to that tree and the history
+behind it - most notably the "parent" commits that preceded it in
+history.
+
+Normally a "commit" has one parent: the previous state of the tree
+before a certain change was made. However, sometimes it can have two
+or more parent commits, in which case we call it a "merge", due to the
+fact that such a commit brings together ("merges") two or more
+previous states represented by other commits.
+
+In other words, while a "tree" represents a particular directory state
+of a working directory, a "commit" represents that state in "time",
+and explains how we got there.
+
+You create a commit object by giving it the tree that describes the
+state at the time of the commit, and a list of parents:
+
+	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
+
+and then giving the reason for the commit on stdin (either through
+redirection from a pipe or file, or by just typing it at the tty).
+
+git-commit-tree will return the name of the object that represents
+that commit, and you should save it away for later use. Normally,
+you'd commit a new `HEAD` state, and while git doesn't care where you
+save the note about that state, in practice we tend to just write the
+result to the file pointed at by `.git/HEAD`, so that we can always see
+what the last committed state was.
+
+Here is an ASCII art by Jon Loeliger that illustrates how
+various pieces fit together.
+
+------------
+
+                     commit-tree
+                      commit obj
+                       +----+
+                       |    |
+                       |    |
+                       V    V
+                    +-----------+
+                    | Object DB |
+                    |  Backing  |
+                    |   Store   |
+                    +-----------+
+                       ^
+           write-tree  |     |
+             tree obj  |     |
+                       |     |  read-tree
+                       |     |  tree obj
+                             V
+                    +-----------+
+                    |   Index   |
+                    |  "cache"  |
+                    +-----------+
+         update-index  ^
+             blob obj  |     |
+                       |     |
+    checkout-index -u  |     |  checkout-index
+             stat      |     |  blob obj
+                             V
+                    +-----------+
+                    |  Working  |
+                    | Directory |
+                    +-----------+
+
+------------
+
+
+6) Examining the data
+~~~~~~~~~~~~~~~~~~~~~
+
+You can examine the data represented in the object database and the
+index with various helper tools. For every object, you can use
+gitlink:git-cat-file[1] to examine details about the
+object:
+
+		git-cat-file -t <objectname>
+
+shows the type of the object, and once you have the type (which is
+usually implicit in where you find the object), you can use
+
+		git-cat-file blob|tree|commit|tag <objectname>
+
+to show its contents. NOTE! Trees have binary content, and as a result
+there is a special helper for showing that content, called
+`git-ls-tree`, which turns the binary content into a more easily
+readable form.
+
+It's especially instructive to look at "commit" objects, since those
+tend to be small and fairly self-explanatory. In particular, if you
+follow the convention of having the top commit name in `.git/HEAD`,
+you can do
+
+		git-cat-file commit HEAD
+
+to see what the top commit was.
+
+7) Merging multiple trees
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Git helps you do a three-way merge, which you can expand to n-way by
+repeating the merge procedure arbitrary times until you finally
+"commit" the state.  The normal situation is that you'd only do one
+three-way merge (two parents), and commit it, but if you like to, you
+can do multiple parents in one go.
+
+To do a three-way merge, you need the two sets of "commit" objects
+that you want to merge, use those to find the closest common parent (a
+third "commit" object), and then use those commit objects to find the
+state of the directory ("tree" object) at these points.
+
+To get the "base" for the merge, you first look up the common parent
+of two commits with
+
+		git-merge-base <commit1> <commit2>
+
+which will return you the commit they are both based on.  You should
+now look up the "tree" objects of those commits, which you can easily
+do with (for example)
+
+		git-cat-file commit <commitname> | head -1
+
+since the tree object information is always the first line in a commit
+object.
+
+Once you know the three trees you are going to merge (the one
+"original" tree, aka the common case, and the two "result" trees, aka
+the branches you want to merge), you do a "merge" read into the
+index. This will complain if it has to throw away your old index contents, so you should
+make sure that you've committed those - in fact you would normally
+always do a merge against your last commit (which should thus match
+what you have in your current index anyway).
+
+To do the merge, do
+
+		git-read-tree -m -u <origtree> <yourtree> <targettree>
+
+which will do all trivial merge operations for you directly in the
+index file, and you can just write the result out with
+`git-write-tree`.
+
+Historical note.  We did not have `-u` facility when this
+section was first written, so we used to warn that
+the merge is done in the index file, not in your
+working tree, and your working tree will not match your
+index after this step.
+This is no longer true.  The above command, thanks to `-u`
+option, updates your working tree with the merge results for
+paths that have been trivially merged.
+
+
+8) Merging multiple trees, continued
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Sadly, many merges aren't trivial. If there are files that have
+been added.moved or removed, or if both branches have modified the
+same file, you will be left with an index tree that contains "merge
+entries" in it. Such an index tree can 'NOT' be written out to a tree
+object, and you will have to resolve any such merge clashes using
+other tools before you can write out the result.
+
+You can examine such index state with `git-ls-files --unmerged`
+command.  An example:
+
+------------------------------------------------
+$ git-read-tree -m $orig HEAD $target
+$ git-ls-files --unmerged
+100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1	hello.c
+100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2	hello.c
+100644 cc44c73eb783565da5831b4d820c962954019b69 3	hello.c
+------------------------------------------------
+
+Each line of the `git-ls-files --unmerged` output begins with
+the blob mode bits, blob SHA1, 'stage number', and the
+filename.  The 'stage number' is git's way to say which tree it
+came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
+tree, and stage3 `$target` tree.
+
+Earlier we said that trivial merges are done inside
+`git-read-tree -m`.  For example, if the file did not change
+from `$orig` to `HEAD` nor `$target`, or if the file changed
+from `$orig` to `HEAD` and `$orig` to `$target` the same way,
+obviously the final outcome is what is in `HEAD`.  What the
+above example shows is that file `hello.c` was changed from
+`$orig` to `HEAD` and `$orig` to `$target` in a different way.
+You could resolve this by running your favorite 3-way merge
+program, e.g.  `diff3` or `merge`, on the blob objects from
+these three stages yourself, like this:
+
+------------------------------------------------
+$ git-cat-file blob 263414f... >hello.c~1
+$ git-cat-file blob 06fa6a2... >hello.c~2
+$ git-cat-file blob cc44c73... >hello.c~3
+$ merge hello.c~2 hello.c~1 hello.c~3
+------------------------------------------------
+
+This would leave the merge result in `hello.c~2` file, along
+with conflict markers if there are conflicts.  After verifying
+the merge result makes sense, you can tell git what the final
+merge result for this file is by:
+
+	mv -f hello.c~2 hello.c
+	git-update-index hello.c
+
+When a path is in unmerged state, running `git-update-index` for
+that path tells git to mark the path resolved.
+
+The above is the description of a git merge at the lowest level,
+to help you understand what conceptually happens under the hood.
+In practice, nobody, not even git itself, uses three `git-cat-file`
+for this.  There is `git-merge-index` program that extracts the
+stages to temporary files and calls a "merge" script on it:
+
+	git-merge-index git-merge-one-file hello.c
+
+and that is what higher level `git resolve` is implemented with.