pack-bitmap-write: sort pseudo-merge commit lookup table in pack order

pack-bitmap-write: sort pseudo-merge commit lookup table in pack order

The pseudo-merge commit lookup table stores each commit's position in
the pack- or pseudo-pack order, and is used to perform a binary search
in order to determine which pseudo-merge(s) a given commit belongs to.

However, the table was previously sorted in lexical order (via
`oid_array_sort()`), causing the binary search to fail.

While this causes pseudo-merge bitmaps to be de-facto broken for fill-in
traversal, there are a couple of important points to keep in mind:

 * Pseudo-merge application during the initial phases of a bitmap-based
   traversal are applied via `cascade_pseudo_merges_1()`. This function
   enumerates the known pseudo-merges and determines if its parents are
   a subset of the traversal roots.

   This is a different path than the fill-in traversal, where we are
   looking for any pseudo-merges which may be satisfied after visiting
   some commit along an object walk, which involves the aforementioned
   (broken) binary search.

   As a consequence, any pseudo-merges we apply at this stage are done
   so correctly.

 * While this bug makes applying pseudo-merges during fill-in traversal
   effectively broken, it does not produce wrong results. Instead of
   applying the *wrong* pseudo-merge, we will simply fail to find
   satisfied pseudo-merges, leaving the traversal to use the existing
   fill-in routines.

Fix this by sorting the table by bit position before writing, matching
the order that the reader's binary search expects.

This does produce a change the on-disk format insofar as the actual code
now complies with the documented format (for more details, refer to:
Documentation/technical/bitmap-format.adoc). Given that this never
worked in the first place, such a change should be OK to perform.

If an out-of-tree implementation of pseudo-merges happened to generate
bitmaps that comply with the documented format, they will continue to be
read and interpreted as normal.

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