Teach git-describe --exact-match to avoid expensive tag searches

Sometimes scripts want (or need) the annotated tag name that exactly
matches a specific commit, or no tag at all.  In such cases it can be
difficult to determine if the output of `git describe $commit` is a
real tag name or a tag+abbreviated commit.  A common idiom is to run
git-describe twice:

  if test $(git describe $commit) = $(git describe --abbrev=0 $commit)
  ...

but this is a huge waste of time if the caller is just going to pick a
different method to describe $commit or abort because it is not exactly
an annotated tag.

Setting the maximum number of candidates to 0 allows the caller to ask
for only a tag that directly points at the supplied commit, or to have
git-describe abort if no such item exists.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/git-describe.txt b/Documentation/git-describe.txt
index 1c3dfb40c..fbb40a291 100644
--- a/Documentation/git-describe.txt
+++ b/Documentation/git-describe.txt
@@ -45,6 +45,11 @@ OPTIONS
 	candidates to describe the input committish consider
 	up to <n> candidates.  Increasing <n> above 10 will take
 	slightly longer but may produce a more accurate result.
+	An <n> of 0 will cause only exact matches to be output.
+
+--exact-match::
+	Only output exact matches (a tag directly references the
+	supplied commit).  This is a synonym for --candidates=0.
 
 --debug::
 	Verbosely display information about the searching strategy