Do not exclude objects in locked packs from bitmap processing

Do not exclude objects in locked packs from bitmap processing

Packfiles having an equivalent .keep file are associated with in-flight
pushes that haven't been completed, with potentially a set of git
objects not yet referenced by a ref.

If the Git client is not up-to-date, it may result in pushing a
packfile, generating a <packfile>.keep on the server, which
may also contain existing commits due to the lack of Git protocol
negotiation in the git-receive-pack.

The Git protocol negotiation is the phase where the client and the
server exchange the list of refs they have for trying to find a common
base and minimise the amount of objects to be transferred.

The repack phase in GC was previously skipping all objects that were
contained in all packfiles having a <packfile>.keep file associated
(aka "locked packfiles"), which did not take into consideration the
fact that excluding the existing commits would have resulted in the
generation of an invalid bitmap file.

The code for excluding the objects in the locked packfiles was written
well before the bitmap was introduced, hence could not consider a use
case that did not exist at that time.

However, when the bitmap was introduced, the exclusion of locked
packfiles was not changed, hence creating a potential problem.
The issue went unnoticed for many years because the bitmap generation
was disabled when JGit noticed any locked packfiles; however, the
bitmaps are enabled again since  Id722e68d9f , and the the issue is now
visible and is impacting the GC repack phase.

Introduce the '--pack-kept-objects' option in GC for including the
objects contained in the locked packfiles during the repack phase,
which is not an issue because of the following:

- If there are any existing commits duplicated in the packfiles
  they will be just considered once anyway because the repack doesn't
  generate duplicates in the output packfile.

- If there are any new commits that do not have any ref pointing to
  them, they will be automatically excluded from the output repacked
  packfile.

The same identical solution is adopted in the C implementation of git
in repack.c.

Because the locked packfile is not pruned, any new commits not pointed
by any refs will remain in the repository and there will not be any
accidental pruning or object loss as it is today before this change.

As a side-effect of this change, it is now potentially possible to still
have duplicate BLOBs after GC when the keep packfile contained existing
objects. However, it is way better to keep the duplication until the
next GC phase rather than omitting existing objects from repacking and,
therefore generating an invalid bitmap and incorrect packfile.

Bug: 582292
Bug: 582455
Change-Id: Ide3445e652fcf256a7912f881cb898897c99b8f8