Fix PackWriterBitmapWalker handling non-existing uninteresting objects

Fix PackWriterBitmapWalker handling non-existing uninteresting objects

When writing new packs it should be allowed to specify objects as "have"
(objects which should not be included in the pack) which do not exist in
the local repository.

This works with the traditional PackWriter, but when PackWriter was
working on a repository with bitmap indexes and used
PackWriterBitmapWalker then this feature was broken. Non-existing "have"
objects lead to MissingObjectExceptions. That broke push and Gerrit
replication. When the replication target had branches unknown to the
replication source then the source repository wanted to build pack files
where "have" included branch-tips which were unknown in the source
repository.

Bug: 427107
Change-Id: I6b6598a1ec49af68aa77ea6f1f06e827982ea4ac
Also-by: Matthias Sohn <matthias.sohn@sap.com>