The find_oldest_filediff_ancestor utility function can now find

ancestor FileDiffs in commit intervals, as well as the oldest

ancestor commit. As such, it has been renamed to

find_ancestor_filediff. Unit tests have been added for both the new

and old behaviour of this function.
A new utility function, find_ancestor_commit_ids, was added to allow

this behaviour. A unit test has been added for this behaviour.

Ran unit tests.