merge: notice local merging of tags and keep it unwrapped

This also updates the autogenerated merge title message from "merge commit X"
to "merge tag X", and its effect can be seen in the changes to the test suite.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/merge.c b/builtin/merge.c
index dffd5ec..6a44b6d 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -441,6 +441,11 @@
 				    sha1_to_hex(branch_head), remote);
 			goto cleanup;
 		}
+		if (!prefixcmp(found_ref, "refs/tags/")) {
+			strbuf_addf(msg, "%s\t\ttag '%s' of .\n",
+				    sha1_to_hex(branch_head), remote);
+			goto cleanup;
+		}
 		if (!prefixcmp(found_ref, "refs/remotes/")) {
 			strbuf_addf(msg, "%s\t\tremote-tracking branch '%s' of .\n",
 				    sha1_to_hex(branch_head), remote);