t5310-pack-bitmaps: fix bogus 'pack-objects to file can use bitmap' test

t5310-pack-bitmaps: fix bogus 'pack-objects to file can use bitmap' test

The test 'pack-objects to file can use bitmap' added in 645c432d61
(pack-objects: use reachability bitmap index when generating
non-stdout pack, 2016-09-10) is silently buggy and doesn't check what
it's supposed to.

In 't5310-pack-bitmaps.sh', the 'list_packed_objects' helper function
does what its name implies by running:

  git show-index <"$1" | cut -d' ' -f2

The test in question invokes this function like this:

  list_packed_objects <packa-$packasha1.idx >packa.objects &&
  list_packed_objects <packb-$packbsha1.idx >packb.objects &&
  test_cmp packa.objects packb.objects

Note how these two callsites don't specify the name of the pack index
file as the function's parameter, but redirect the function's standard
input from it.  This triggers an error message from the shell, as it
has no filename to redirect from in the function, but this error is
ignored, because it happens upstream of a pipe.  Consequently, both
invocations produce empty 'pack{a,b}.objects' files, and the
subsequent 'test_cmp' happily finds those two empty files identical.

Fix these two 'list_packed_objects' invocations by specifying the pack
index files as parameters.  Furthermore, eliminate the pipe in that
function by replacing it with an &&-chained pair of commands using an
intermediate file, so a failure of 'git show-index' or the shell
redirection will fail the test.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>