Add support for git config repack.packKeptObjects

Add support for git config repack.packKeptObjects

Change Ide3445e652 introduced the `--pack-kept-objects` option to GC for
including the objects contained in the locked packfiles during the
repack phase.

Whilst this allowed to explicitly pass a command line argument to the
jgit gc program, it did not allow the option to be read from
configuration.

Allow the pack kept objects option to be configured exactly as C-Git
documents [1], by introducing a new `repack.packKeptObjects`
configuration.

`repack.packKeptObjects` defaults to `true`, when the
`pack.buildBitmaps` is `true` (which is the default case), `false`
otherwise.

[1] https://git-scm.com/docs/git-config#Documentation/git-config.txt-repackpackKeptObjects

Bug: 582292
Change-Id: Ia931667277410d71bc079d27c097a57094299840