Make GC more robust against corrupt reflogs

Make GC more robust against corrupt reflogs

With JGit it is possible to write reflog entries where new objectid and
old objectid is null. Such reflogs cause FileRepository GC to crash
because it doesn't expect the new objectid to be null. One case where
this happened is in Gerrit's allProjects repo. In the same way as we
expect the old objectid to be potentially null we should also ignore
null values in the new objectid column.

Change-Id: Icf666c7ef803179b84306ca8deb602369b8df16e