refs: move ref name helpers around

strbuf_branchname(), strbuf_check_{branch,tag}_ref() are helper
functions to deal with branch and tag names, and the fact that they
happen to use strbuf to hold the name of a branch or a tag is not
essential.  These functions fit better in the refs API than strbuf
API, the latter of which is about string manipulations.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -447,17 +447,6 @@ static int parse_msg_arg(const struct option *opt, const char *arg, int unset)
 	return 0;
 }
 
-static int strbuf_check_tag_ref(struct strbuf *sb, const char *name)
-{
-	if (name[0] == '-')
-		return -1;
-
-	strbuf_reset(sb);
-	strbuf_addf(sb, "refs/tags/%s", name);
-
-	return check_refname_format(sb->buf, 0);
-}
-
 int cmd_tag(int argc,
 	    const char **argv,
 	    const char *prefix,