RevWalk: Traverse all parents of UNINTERESTING commits

RevWalk: Traverse all parents of UNINTERESTING commits

When firstParent is set, RevWalk traverses only the first parent of a
commit, even though that commit is UNINTERESTING. Since we want the
maximal UNINTERESTING set, we shouldn't prune any parents here. This
issue is apparent only when some of the commits being traversed are
unparsed, since walker.carryFlagsImpl() propagates the UNINTERESTING
flag to all parsed ancestors, masking the issue.

Therefore teach RevWalk to traverse all parents when a commit is
UNINTERESTING and not only the first parent. Since this issue is
masked by commit parsing, also test situations when the commits
involved are unparsed.

Signed-off-by: Alex Spradlin <alexaspradlin@google.com>
Change-Id: I95e2ad9ae8f1f50fbecae674367ee7e0855519b1