Documentation/technical: describe pseudo-merge bitmaps format

Documentation/technical: describe pseudo-merge bitmaps format

Prepare to implement pseudo-merge bitmaps over the next several commits
by first describing the serialization format which will store the new
pseudo-merge bitmaps themselves.

This format is implemented as an optional extension within the bitmap v1
format, making it compatible with previous versions of Git, as well as
the original .bitmap implementation within JGit.

The format is described in detail in the patch contents below, but the
high-level description is as follows:

  - An array of pseudo-merge bitmaps, each containing a pair of EWAH
    bitmaps: one describing the set of pseudo-merge "parents", and
    another describing the set of object(s) reachable from those
    parents.

  - A lookup table to determine which pseudo-merge(s) a given commit
    appears in. An optional extended lookup table follows when there is
    at least one commit which appears in multiple pseudo-merge groups.

  - Trailing metadata, including the number of pseudo-merge(s), number
    of unique parents, the offset within the .bitmap file for the
    pseudo-merge commit lookup table, and the size of the optional
    extension itself.

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