repack: check error writing to pack-objects subprocess

repack: check error writing to pack-objects subprocess

When "git repack" repacks promisor objects, it starts a pack-objects
subprocess and uses xwrite() to send object names over the pipe to
it, but without any error checking.  An I/O error or short write
(even though a short write is unlikely for such a small amount of
data) can result in a packfile that lacks certain objects we wanted
to put in there, leading to a silent repository corruption.

Use write_in_full(), instead of xwrite(), to mitigate short write
risks, check errors from it, and abort if we see a failure.

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