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>