Marking new start commits and iterating a RevWalk is only safe if the
RevWalk was previously reset. Otherwise its internal graph state may
be pointing to some other commit completely unrelated to what the
caller is intending to walk. In the case that the RevWalk previously
had findMergedInto called on it, this manifests as an
UnsupportedOperationException from AlwaysEmptyQueue.
Searched for all instances of "markStart" and ensured they were either
preceded by reset/resetRetain or the RevWalk was brand new.
Change-Id: I1fc34e7b8783921cf0feef1fab9f546700d2fbed
676e09881f