This change removes the last bit of deprecated API for 5.0: the old

get_orig_commit_id method. This was more complex than the rest because

the mercurial backend still used it in its diff parsers (both mercurial

and git modes).
I've updated the hg parsers to instead set the parent commit ID on the

parsed diff change, as well as set the uses_commit_ids_as_revisions

flag.
This also required updating a unit test which wasn't set up quite right.

Now that commits and parent commits are done per file instead of per

diff (as per all the other machinery in create_filediffs), we can't

have a parent diff that specifies different files than the diff and

expect the parent diff commit ID to get set correctly.
Testing Done:

Ran unit tests.