git-repack: avoid redirecting stderr into git-pack-objects

git-repack: avoid redirecting stderr into git-pack-objects

We are trying to catch error condition of git-rev-list and cause
the downstream pack-objects to barf, but if you run rev-list
with anything that mucks with its stderr (such as GIT_TRACE),
any stderr output would cause the pipeline to fail.

[jc: originally from Matthias Lederhofer, with a reworded error message.]

Signed-off-by: Junio C Hamano <junkio@cox.net>