t5310: increase the number of bitmapped commits

The bitmap index we compute in t5310 has only 20 commits in it. This gives poor coverage of bitmap_writer_select_commits(), which simply writes a bitmap for everything when there are fewer than 100 commits. Let's bump the number of commits in the test to cover the more complex code paths (this does drop coverage of the individual lines of the trivial path, but the complex path does everything it does and more). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>