Merge branch 'jc/forbid-head-as-tagname'

"git tag" has been taught to refuse to create refs/tags/HEAD
as such a tag will be confusing in the context of UI provided by
the Git Porcelain commands.

* jc/forbid-head-as-tagname:
  tag: "git tag" refuses to use HEAD as a tagname
  t5604: do not expect that HEAD can be a valid tagname
  refs: drop strbuf_ prefix from helpers
  refs: move ref name helpers around

builtin/merge.c

@@ -498,7 +498,7 @@ static void merge_name(const char *remote, struct strbuf *msg)
 	char *found_ref = NULL;
 	int len, early;
 
-	strbuf_branchname(&bname, remote, 0);
+	copy_branchname(&bname, remote, 0);
 	remote = bname.buf;
 
 	oidclr(&branch_head, the_repository->hash_algo);