git push: say that --tag can't be used with --all or --mirror in help text

This replaces an earlier patch by Björn Gustavsson,

  Message-ID: <4AD75029.1010109@gmail.com>
Signed-off-by: Nしらいし ななこ <nanako3@lavabit.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-push.c

@@ -181,7 +181,7 @@ int cmd_push(int argc, const char **argv, const char *prefix)
 		OPT_BIT( 0 , "all", &flags, "push all refs", TRANSPORT_PUSH_ALL),
 		OPT_BIT( 0 , "mirror", &flags, "mirror all refs",
 			    (TRANSPORT_PUSH_MIRROR|TRANSPORT_PUSH_FORCE)),
-		OPT_BOOLEAN( 0 , "tags", &tags, "push tags"),
+		OPT_BOOLEAN( 0 , "tags", &tags, "push tags (can't be used with --all or --mirror"),
 		OPT_BIT('n' , "dry-run", &flags, "dry run", TRANSPORT_PUSH_DRY_RUN),
 		OPT_BIT( 0,  "porcelain", &flags, "machine-readable output", TRANSPORT_PUSH_PORCELAIN),
 		OPT_BIT('f', "force", &flags, "force updates", TRANSPORT_PUSH_FORCE),