Allow 'MonotoneHistoryProvider' to get file contents at the
requested revision
This allows NetBeans to provide diffs for us using the 'Diff To Current' and
'Diff To Previous' dropdown.
Added 'getFileOf' and 'parents' to 'Automate' which is used to provide the
functionality.
Currently our 'automate.log' returns revisions by looking back in history
so the latest commit is 1st, however, I don't think the diff results were
correct in the NetBeans 'History' tab. Reversing the order we return the
'HistoryEntry's i seems to fix it. I also can't find anything in the NetBeans
API to suggest which order they want them in.
Really though, we should pass the revision IDs through `mtn au toposort`
if that's how NetBeans really wants them returned, but for now it's ok.