t5310: correctly remove bitmaps for jgit test

t5310: correctly remove bitmaps for jgit test

We use "jgit gc" to generate a pack bitmap file, and then make sure our
implementation can read it. To prepare the repo before running jgit, we
try to "rm -f" any existing bitmap files. But we got the path wrong;
we're in a bare repo, so looking in ".git/" finds nothing. Our "rm"
doesn't complain because of the "-f", and when we run "rev-list" there
are two bitmap files (ours and jgit's).

Our reader implementation will ignore one of the bitmap files, but it's
likely non-deterministic which one we will use. We'd prefer the one with
the more recent timestamp (just because of the way the packed_git list
is sorted), but in most test runs they'd have identical timestamps.

So this was probably actually testing something useful about 50% of the
time, and other half just testing that we could read our own bitmaps
(which is covered elsewhere).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>