pack-bitmap-write.c: select pseudo-merges even for small bitmaps

pack-bitmap-write.c: select pseudo-merges even for small bitmaps

Ordinarily, the pack-bitmap machinery will select some subset of
reachable commits to receive bitmaps. But when there are fewer than 100
commits indexed in the first place, they will all receive bitmaps as a
special case.

When this happens, pseudo-merges are not generated, making it impossible
to test pseudo-merge corner cases with fewer than 100 commits.

Select pseudo-merges even for bitmaps with fewer than 100 commits to
make such testing easier. In practice, this should not make a difference
to non-testing bitmaps, as they are unlikely to be used when a
repository has so few commits to begin with.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>