Documentation: emphasize when git merge terminates early

A merge-based operation in git can fail in two ways: one that
stops before touching anything, or one that goes ahead and
results in conflicts.

As the 'git merge' manual explains:

| A merge is always between the current `HEAD` and one or more
| commits (usually, branch head or tag), and the index file must
| match the tree of `HEAD` commit (i.e. the contents of the last commit)
| when it starts out.

Unfortunately, the placement of this sentence makes it easy to
skip over, and its formulation leaves the important point, that
any other attempted merge will be gracefully aborted, unspoken.

So give this point its own section and expand upon it.

Probably this could be simplified somewhat: after all, a change
registered in the index is just a special kind of local
uncommited change, so the second added paragraph is only a
special case of the first.  It seemed more helpful to be explicit
here.

Inspired by <http://gitster.livejournal.com/25801.html>.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>

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