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>

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

builtin-describe.c

@@ -174,6 +174,8 @@ static void describe(const char *arg, int last_one)
 		return;
 	}
 
+	if (!max_candidates)
+		die("no tag exactly matches '%s'", sha1_to_hex(cmit->object.sha1));
 	if (debug)
 		fprintf(stderr, "searching to describe %s\n", arg);
 
@@ -270,6 +272,8 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 		OPT_BOOLEAN(0, "all",        &all, "use any ref in .git/refs"),
 		OPT_BOOLEAN(0, "tags",       &tags, "use any tag in .git/refs/tags"),
 		OPT__ABBREV(&abbrev),
+		OPT_SET_INT(0, "exact-match", &max_candidates,
+			    "only output exact matches", 0),
 		OPT_INTEGER(0, "candidates", &max_candidates,
 			    "consider <n> most recent tags (default: 10)"),
 		OPT_STRING(0, "match",       &pattern, "pattern",
@@ -278,8 +282,8 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 	};
 
 	argc = parse_options(argc, argv, options, describe_usage, 0);
-	if (max_candidates < 1)
-		max_candidates = 1;
+	if (max_candidates < 0)
+		max_candidates = 0;
 	else if (max_candidates > MAX_TAGS)
 		max_candidates = MAX_TAGS;