Convert a hardcoded constant buffer size to a use of GIT_MAX_HEXSZ, and
use parse_oid_hex to reduce the dependency on the size of the hash.
This function is a caller of sha1_array_append, which will be converted
later.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>