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>