commit,merge,tag: describe -m likewise

This also removes the superfluous "specify" and rewords the misleading
"if any" which sounds as if omitting "-m" would omit the merge commit
message. (It means "if a merge commit is created at all".)

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/tag.c b/builtin/tag.c
index 6408171..246a2bc 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -376,8 +376,8 @@
 		OPT_GROUP("Tag creation options"),
 		OPT_BOOLEAN('a', NULL, &annotate,
 					"annotated tag, needs a message"),
-		OPT_CALLBACK('m', NULL, &msg, "msg",
-			     "message for the tag", parse_msg_arg),
+		OPT_CALLBACK('m', NULL, &msg, "MESSAGE",
+			     "tag message", parse_msg_arg),
 		OPT_FILENAME('F', NULL, &msgfile, "read message from file"),
 		OPT_BOOLEAN('s', NULL, &sign, "annotated and GPG-signed tag"),
 		OPT_STRING('u', NULL, &keyid, "key-id",