PackBitmapIndex: Reduce memory usage in GC

PackBitmapIndex: Reduce memory usage in GC

Currently, the garbage collection is consistently failing for some large
repositories in the building bitmap phase, e.g.Linux-MSM project:
https://source.codeaurora.org/quic/la/kernel/msm-3.18

Historically, bitmap index creation happened in 3 phases:
1. Select the commits to which bitmaps should be attached.
2. Create all bitmaps for these commits, stored in uncompressed format
in the PackBitmapIndexBuilder.
3. Deltify the bitmaps and write them to disk.

We investigated the process. For phase 2 it's most efficient to create
bitmaps starting with oldest commit and moving to the newest commit,
because the newer commits are able to reuse the work for the old ones.
But for bitmap deltification in phase 3, it's better when a newer
commit's bitmap is the base, and the current disk format writes bitmaps
out for the newest commits first.

This change introduces a new collection to hold the deltified and
compressed representations of the bitmaps, keeping a smaller subset of
commits in the PackBitmapIndexBuilder to help make the bitmap index
creation more memory efficient.

And in this commit, we're setting DISTANCE_THRESHOLD to 0 in the
PackWriterBitmapPreparer, which means the garbage collection will not
have much behavoir change and will still use as much memory as before.

Change-Id: I6ec2c3e8dde11805af47874d67d33cf1ef83660e
Signed-off-by: Yunjie Li <yunjieli@google.com>