When marking commits as uninteresting don't care if the tree exists

When during an ObjectWalk commits are marked as uninteresting we should
be tolerant against the situation that the commit exists in the repo but
the referenced tree is not exisiting. Since commit
c4797fe98655b3d52d0a90ba44fce6e053db3b8b we are throwing
MissingObjectException in such a case. This semantic differs from native
git behaviour and may cause push operations to fail while they would
work in native git. See:
http://dev.eclipse.org/mhonarc/lists/egit-dev/msg03585.html

Bug: 445744
Change-Id: Ib7dec10fd2ef1adbb8adbabb9d3d5a64e554286a