builtin/receive-pack: convert portions to struct object_id

builtin/receive-pack: convert portions to struct object_id

Convert some hardcoded constants into uses of parse_oid_hex.
Additionally, convert all uses of struct command, and miscellaneous
other functions necessary for that.  This work is necessary to be able
to convert sha1_array_append later on.

To avoid needing to specify a constant, reject shallow lines with the
wrong length instead of simply ignoring them.

Note that in queue_command we are guaranteed to have a NUL-terminated
buffer or at least one byte of overflow that we can safely read, so the
linelen check can be elided.  We would die in such a case, but not read
invalid memory.

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