send pack check for failure to send revisions list

send pack check for failure to send revisions list

When passing the revisions list to pack-objects we do not check for
errors nor short writes.  Introduce a new write_in_full which will
handle short writes and report errors to the caller.  Use this to
short cut the send on failure, allowing us to wait for and report
the child in case the failure is its fault.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>