strbuf_check_branch_ref(): a helper to check a refname for a branch

strbuf_check_branch_ref(): a helper to check a refname for a branch

This allows a common calling sequence

	strbuf_branchname(&ref, name);
	strbuf_splice(&ref, 0, 0, "refs/heads/", 11);
	if (check_ref_format(ref.buf))
		die(...);

to be refactored into

	if (strbuf_check_branch_ref(&ref, name))
		die(...);

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