pseudo-merge: implement support for finding existing merges

This patch implements support for reusing existing pseudo-merge commits
when writing bitmaps when there is an existing pseudo-merge bitmap which
has exactly the same set of parents as one that we are about to write.

Note that unstable pseudo-merges are likely to change between
consecutive repacks, and so are generally poor candidates for reuse.
However, stable pseudo-merges (see the configuration option
'bitmapPseudoMerge.<name>.stableThreshold') are by definition unlikely
to change between runs (as they represent long-running branches).

Because there is no index from a *set* of pseudo-merge parents to a
matching pseudo-merge bitmap, we have to construct the bitmap
corresponding to the set of parents for each pending pseudo-merge commit
and see if a matching bitmap exists.

This is technically quadratic in the number of pseudo-merges, but is OK
in practice for a couple of reasons:

- non-matching pseudo-merge bitmaps are rejected quickly as soon as
they differ in a single bit

- already-matched pseudo-merge bitmaps are discarded from subsequent
rounds of search

- the number of pseudo-merges is generally small, even for large
repositories

In order to do this, implement (a) a function that finds a matching
pseudo-merge given some uncompressed bitset describing its parents, (b)
a function that computes the bitset of parents for a given pseudo-merge
commit, and (c) call that function before computing the set of reachable
objects for some pending pseudo-merge.

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