refs: drop strbuf_ prefix from helpers
The helper functions (strbuf_branchname, strbuf_check_branch_ref,
and strbuf_check_tag_ref) are about handling branch and tag names,
and it is a non-essential fact that these functions use strbuf to
hold these names. Rename them to make it clarify that these are
more about "ref".
Signed-off-by: Junio C Hamano <gitster@pobox.com>