Git.pm: localise $? in command_close_bidi_pipe()

Git.pm: localise $? in command_close_bidi_pipe()

Git::DESTROY calls _close_cat_blob and _close_hash_and_insert_object,
which in turn call command_close_bidi_pipe, which calls waitpid, which
alters $?. If this happens during global destruction, it may alter the
program's exit status unexpectedly. Making $? local to the function
solves the problem.

(The problem was discovered due to a failure of test #8 in
t9106-git-svn-commit-diff-clobber.sh.)

Signed-off-by: Abhijit Menon-Sen <ams@toroid.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>