Merge branch 'jk/unavailable-can-be-missing'

Code clean-up to turn history traversal more robust in a
semi-corrupt repository.

* jk/unavailable-can-be-missing:
mark_parents_uninteresting(): avoid most allocation
mark_parents_uninteresting(): replace list with stack
mark_parents_uninteresting(): drop missing object check
mark_tree_contents_uninteresting(): drop missing object check

diff --cc revision.c
Simple merge