Shadow commits in the RevWalk in the UploadPack object may cause the
UNINTERESTING flag not being carried over to their parents commits since
they were marked NO_PARENTS during the assumeShallow or
initializeShallowCommits call.
A new RevWalk needs to be created for this reason, but instead of
creating a new RevWalk from Repository, we can reuse the ObjectReader in
the RevWalk of UploadPack to load objects.
Change-Id: Ic3fee0512d35b4f555c60e696a880f8b192e4439
Signed-off-by: Zhen Chen <czhen@google.com>