fast-import: convert to struct object_id

Convert the remaining parts of fast-import.c to use struct object_id.
Convert several instances of get_sha1_hex to parse_oid_hex to avoid
needing to specify constants.  Convert other hardcoded values to named
constants.  Finally, use the is_empty_tree_oid function instead of a
direct comparison against a fixed string.

Note that the odd computation with GIT_MAX_HEXSZ is due to the insertion
of a slash between every two hex digits in the path, plus one for the
terminating NUL.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>