t7900: fix a flaky test due to git-repack always regenerating MIDX
When a supposedly no-op "git repack" runs across a second boundary, because the command always touches the MIDX file and updates its timestamp, "ls -l $GIT_DIR/objects/pack/" before and after the operation can change, which causes such a test to fail. Only compare the *.pack files in the directory before and after the operation to work around this flakyness. Arguably, git-repack(1) should learn to not rewrite the MIDX in case we know it is already up-to-date. But this is not a new problem introduced via the new geometric maintenance task, so for now it should be good enough to paper over the issue. Signed-off-by: Patrick Steinhardt <ps@pks.im> [jc: taken from diff to v4 from v3 that was already merged to 'next'] Signed-off-by: Junio C Hamano <gitster@pobox.com>