Convert the exclude_sha1 member of struct untracked_cache_dir and rename
it to exclude_oid. Eliminate several hard-coded integral constants, and
update a function name that referred to SHA-1.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>