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>