unpack: replace xwrite() loop with write_in_full()

unpack: replace xwrite() loop with write_in_full()

We have two packfile stream consumers, index-pack and
unpack-objects, that allow excess payload after the packfile stream
data. Their code to relay excess data hasn't changed significantly
since their original implementation that appeared in 67e5a5ec
(git-unpack-objects: re-write to read from stdin, 2005-06-28) and
9bee2478 (mimic unpack-objects when --stdin is used with index-pack,
2006-10-25).

These code blocks contain hand-rolled loops using xwrite(), written
before our write_in_full() helper existed. This helper now provides
the same functionality.

Replace these loops with write_in_full() for shorter, clearer
code. Update related variables accordingly.

Signed-off-by: Junio C Hamano <gitster@pobox.com>