t7900: fix a flaky test due to git-repack always regenerating MIDX

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>