pack-redundant: fix crash when one packfile in repo

Command `git pack-redundant --all` will crash if there is only one
packfile in the repository.  This is because, if there is only one
packfile in local_packs, `cmp_local_packs` will do nothing and will
leave `pl->unique_objects` as uninitialized.

Also add testcases for repository with no packfile and one packfile
in t5323.

Reported-by: Daniel C. Klauer <daniel.c.klauer@web.de>
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jiangxin

builtin/pack-redundant.c

@@ -473,6 +473,12 @@ static void cmp_local_packs(void)
 {
 	struct pack_list *subset, *pl = local_packs;
 
+	/* only one packfile */
+	if (!pl->next) {
+		llist_init(&pl->unique_objects);
+		return;
+	}
+
 	while ((subset = pl)) {
 		while ((subset = subset->next))
 			cmp_two_packs(pl, subset);

t/t5323-pack-redundant.sh

@@ -112,19 +112,28 @@ test_expect_success 'setup master repo' '
 	create_commits_in "$master_repo" A B C D E F G H I J K L M N O P Q R
 '
 
+test_expect_success 'master: pack-redundant works with no packfile' '
+	(
+		cd "$master_repo" &&
+		cat >expect <<-EOF &&
+			fatal: Zero packs found!
+			EOF
+		test_must_fail git pack-redundant --all >actual 2>&1 &&
+		test_cmp expect actual
+	)
+'
+
 #############################################################################
 # Chart of packs and objects for this test case
 #
 #         | T A B C D E F G H I J K L M N O P Q R
 #     ----+--------------------------------------
 #     P1  | x x x x x x x                       x
-#     P2  |     x x x x   x x x
-#     P3  |             x     x x x x x
 #     ----+--------------------------------------
-#     ALL | x x x x x x x x x x x x x x         x
+#     ALL | x x x x x x x                       x
 #
 #############################################################################
-test_expect_success 'master: no redundant for pack 1, 2, 3' '
+test_expect_success 'master: pack-redundant works with one packfile' '
 	create_pack_in "$master_repo" P1 <<-EOF &&
 		$T
 		$A
@@ -135,6 +144,26 @@ test_expect_success 'master: no redundant for pack 1, 2, 3' '
 		$F
 		$R
 		EOF
+	(
+		cd "$master_repo" &&
+		git pack-redundant --all >out &&
+		test_must_be_empty out
+	)
+'
+
+#############################################################################
+# Chart of packs and objects for this test case
+#
+#         | T A B C D E F G H I J K L M N O P Q R
+#     ----+--------------------------------------
+#     P1  | x x x x x x x                       x
+#     P2  |     x x x x   x x x
+#     P3  |             x     x x x x x
+#     ----+--------------------------------------
+#     ALL | x x x x x x x x x x x x x x         x
+#
+#############################################################################
+test_expect_success 'master: no redundant for pack 1, 2, 3' '
 	create_pack_in "$master_repo" P2 <<-EOF &&
 		$B
 		$C