git-repack: create new packs inside $GIT_DIR, not cwd

git-repack: create new packs inside $GIT_DIR, not cwd

Avoid failing when cwd is !writable by writing the
packfiles in $GIT_DIR, which is more in line with other commands.

Without this, git-repack was failing when run from crontab
by non-root user accounts. For large repositories, this
also makes the mv operation a lot cheaper, and avoids leaving
temp packfiles around the fs upon failure.

Signed-off-by: Martin Langhoff <martin@catalyst.net.nz>
Signed-off-by: Junio C Hamano <junkio@cox.net>