t: add basic bitmap functionality tests

t: add basic bitmap functionality tests

Now that we can read and write bitmaps, we can exercise them
with some basic functionality tests. These tests aren't
particularly useful for seeing the benefit, as the test
repo is too small for it to make a difference. However, we
can at least check that using bitmaps does not break anything.

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