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

branch.c

@@ -372,7 +372,7 @@ int read_branch_desc(struct strbuf *buf, const char *branch_name)
  */
 int validate_branchname(const char *name, struct strbuf *ref)
 {
-	if (strbuf_check_branch_ref(ref, name)) {
+	if (check_branch_ref(ref, name)) {
 		int code = die_message(_("'%s' is not a valid branch name"), name);
 		advise_if_enabled(ADVICE_REF_SYNTAX,
 				  _("See `man git check-ref-format`"));