t5310 broken under Mac OS

One test case fails here, but I am to tired to dig further.

ok 42 - pack reuse respects --incremental

expecting success:
     git repack -ad &&
     git rev-list --use-bitmap-index --count --all >expect &&
     bitmap=$(ls .git/objects/pack/*.bitmap) &&
     test_when_finished "rm -f $bitmap" &&
     head -c 512 <$bitmap >$bitmap.tmp &&
     mv $bitmap.tmp $bitmap &&
     git rev-list --use-bitmap-index --count --all >actual 2>stderr &&
     test_cmp expect actual &&
     test_i18ngrep corrupt stderr

override r--r--r--  tb/staff for .git/objects/pack/pack-8886db3fce4f9657c1a43fee7d3ea4f2a4b5be2d.bitmap? (y/n [n]) not overwritten error: 'grep corrupt stderr' didn't find a match in: <File 'stderr' is empty> not ok 43 - truncated bitmap fails gracefully # #        git repack -ad && #        git rev-list --use-bitmap-index --count --all >expect && #        bitmap=$(ls .git/objects/pack/*.bitmap) && #        test_when_finished "rm -f $bitmap" && #        head -c 512 <$bitmap >$bitmap.tmp && #        mv $bitmap.tmp $bitmap && #        git rev-list --use-bitmap-index --count --all >actual 2>stderr && #        test_cmp expect actual && #        test_i18ngrep corrupt stderr #

# failed 1 among 43 test(s) 1..43