If abbrev is set to zero in git-describe, don't add the unique suffix

When on a non-tag commit, git-describe normally outputs descriptions of
the form
  v1.0.0-g1234567890
Some scripts (for example the update hook script) might just want to
know the name of the nearest tag, so they then have to do
 x=$(git-describe HEAD | sed 's/-g*//')
This is costly, but more importantly is fragile as it is relying on the
output format of git-describe, which we would then have to maintain
forever.

This patch adds support for setting the --abbrev option to zero.  In
that case git-describe does as it always has, but outputs only the
nearest found tag instead of a completely unique name.  This means that
scripts would not have to parse the output format and won't need
changing if the git-describe suffix is ever changed.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-describe.c

@@ -188,8 +188,11 @@ static void describe(const char *arg, int last_one)
 				sha1_to_hex(gave_up_on->object.sha1));
 		}
 	}
-	printf("%s-g%s\n", all_matches[0].name->path,
-		   find_unique_abbrev(cmit->object.sha1, abbrev));
+	if (abbrev == 0)
+		printf("%s\n", all_matches[0].name->path );
+	else
+		printf("%s-g%s\n", all_matches[0].name->path,
+			   find_unique_abbrev(cmit->object.sha1, abbrev));
 
 	if (!last_one)
 		clear_commit_marks(cmit, -1);
@@ -212,7 +215,7 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 			tags = 1;
 		else if (!strncmp(arg, "--abbrev=", 9)) {
 			abbrev = strtoul(arg + 9, NULL, 10);
-			if (abbrev < MINIMUM_ABBREV || 40 < abbrev)
+			if (abbrev != 0 && (abbrev < MINIMUM_ABBREV || 40 < abbrev))
 				abbrev = DEFAULT_ABBREV;
 		}
 		else if (!strncmp(arg, "--candidates=", 13)) {