pseudo-merge: implement support for reading pseudo-merge commits

Implement the basic API for reading pseudo-merge bitmaps, which consists
of four basic functions:

  - pseudo_merge_bitmap()
  - use_pseudo_merge()
  - apply_pseudo_merges_for_commit()
  - cascade_pseudo_merges()

These functions are all documented in pseudo-merge.h, but their rough
descriptions are as follows:

  - pseudo_merge_bitmap() reads and inflates the objects EWAH bitmap for
    a given pseudo-merge

  - use_pseudo_merge() does the same as pseudo_merge_bitmap(), but on
    the commits EWAH bitmap, not the objects bitmap

  - apply_pseudo_merges_for_commit() applies all satisfied pseudo-merge
    commits for a given result set, and cascades any yet-unsatisfied
    pseudo-merges if any were applied in the previous step

  - cascade_pseudo_merges() applies all pseudo-merges which are
    satisfied but have not been previously applied, repeating this
    process until no more pseudo-merges can be applied

The core of the API is the latter two functions, which are responsible
for applying pseudo-merges during the object traversal implemented in
the pack-bitmap machinery.

The other two functions (pseudo_merge_bitmap(), and use_pseudo_merge())
are low-level ways to interact with the pseudo-merge machinery, which
will be useful in future commits.

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

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