describe: fix --contains when a tag is given as input

"git describe" takes a commit and gives it a name based on tags in
its neighbourhood.  The command does take a commit-ish but when
given a tag that points at a commit, it should dereference the tag
before computing the name for the commit.

As the whole processing is internally delegated to name-rev, if we
unwrap tags down to the underlying commit when invoking name-rev, it
will make the name-rev issue an error message based on the unwrapped
object name (i.e. either 40-hex object name, or "$tag^0") that is
different from what the end-user gave to the command when the commit
cannot be described.  Introduce an internal option --peel-tag to the
name-rev to tell it to unwrap a tag in its input from the command
line.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/describe.c

@@ -446,7 +446,8 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 		struct argv_array args;
 
 		argv_array_init(&args);
-		argv_array_pushl(&args, "name-rev", "--name-only", "--no-undefined",
+		argv_array_pushl(&args, "name-rev",
+				 "--peel-tag", "--name-only", "--no-undefined",
 				 NULL);
 		if (always)
 			argv_array_push(&args, "--always");

builtin/name-rev.c

@@ -307,7 +307,7 @@ static void name_rev_line(char *p, struct name_ref_data *data)
 int cmd_name_rev(int argc, const char **argv, const char *prefix)
 {
 	struct object_array revs = OBJECT_ARRAY_INIT;
-	int all = 0, transform_stdin = 0, allow_undefined = 1, always = 0;
+	int all = 0, transform_stdin = 0, allow_undefined = 1, always = 0, peel_tag = 0;
 	struct name_ref_data data = { 0, 0, NULL };
 	struct option opts[] = {
 		OPT_BOOLEAN(0, "name-only", &data.name_only, N_("print only names (no SHA-1)")),
@@ -320,6 +320,12 @@ int cmd_name_rev(int argc, const char **argv, const char *prefix)
 		OPT_BOOLEAN(0, "undefined", &allow_undefined, N_("allow to print `undefined` names")),
 		OPT_BOOLEAN(0, "always",     &always,
 			   N_("show abbreviated commit object as fallback")),
+		{
+			/* A Hidden OPT_BOOL */
+			OPTION_SET_INT, 0, "peel-tag", &peel_tag, NULL,
+			N_("dereference tags in the input (internal use)"),
+			PARSE_OPT_NOARG | PARSE_OPT_HIDDEN, NULL, 1,
+		},
 		OPT_END(),
 	};
 
@@ -361,6 +367,15 @@ int cmd_name_rev(int argc, const char **argv, const char *prefix)
 			if (cutoff > commit->date)
 				cutoff = commit->date;
 		}
+
+		if (peel_tag) {
+			if (!commit) {
+				fprintf(stderr, "Could not get commit for %s. Skipping.\n",
+					*argv);
+				continue;
+			}
+			object = (struct object *)commit;
+		}
 		add_object_array(object, *argv, &revs);
 	}
 

t/t6120-describe.sh

@@ -186,4 +186,16 @@ test_expect_success 'name-rev with exact tags' '
 	test_cmp expect actual
 '
 
+test_expect_success 'describe --contains with the exact tags' '
+	echo "A^0" >expect &&
+	tag_object=$(git rev-parse refs/tags/A) &&
+	git describe --contains $tag_object >actual &&
+	test_cmp expect actual &&
+
+	echo "A^0" >expect &&
+	tagged_commit=$(git rev-parse "refs/tags/A^0") &&
+	git describe --contains $tagged_commit >actual &&
+	test_cmp expect actual
+'
+
 test_done