tag: change --point-at to default to HEAD

Change the --points-at option to default to HEAD for consistency with
its siblings --contains, --merged etc. which default to
HEAD. Previously we'd get:

    $ git tag --points-at 2>&1 | head -n 1
    error: option `points-at' requires a value

This changes behavior added in commit ae7706b (tag: add --points-at
list option, 2012-02-08).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-tag.txt

@@ -139,7 +139,8 @@ This option is only applicable when listing tags without annotation lines.
 	commit (`HEAD` if not specified), incompatible with `--merged`.
 
 --points-at <object>::
-	Only list tags of the given object. Implies `--list`.
+	Only list tags of the given object (HEAD if not
+	specified). Implies `--list`.
 
 -m <msg>::
 --message=<msg>::

builtin/tag.c

@@ -431,7 +431,8 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 			     N_("field name to sort on"), &parse_opt_ref_sorting),
 		{
 			OPTION_CALLBACK, 0, "points-at", &filter.points_at, N_("object"),
-			N_("print only tags of the object"), 0, parse_opt_object_name
+			N_("print only tags of the object"), PARSE_OPT_LASTARG_DEFAULT,
+			parse_opt_object_name, (intptr_t) "HEAD"
 		},
 		OPT_STRING(  0 , "format", &format, N_("format"), N_("format to use for the output")),
 		OPT_BOOL('i', "ignore-case", &icase, N_("sorting and filtering are case insensitive")),

t/t7004-tag.sh

@@ -1534,7 +1534,8 @@ do
 	"
 	test_expect_success "Doing 'git tag --list-like $option <commit> <pattern> is permitted" "
 		git tag -n $option HEAD HEAD &&
-		git tag $option HEAD HEAD
+		git tag $option HEAD HEAD &&
+		git tag $option
 	"
 done
 
@@ -1546,6 +1547,12 @@ test_expect_success '--points-at can be used in non-list mode' '
 	test_cmp expect actual
 '
 
+test_expect_success '--points-at is a synonym for --points-at HEAD' '
+	echo v4.0 >expect &&
+	git tag --points-at >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success '--points-at finds lightweight tags' '
 	echo v4.0 >expect &&
 	git tag --points-at v4.0 >actual &&