We can now generate the pre-patch version of a file that is modified

multiple times in a commit history. This allows the diffviewer to show

side-by-side diffs in these cases, however this behaviour is not correct

as it will show files with the same name multiple times when it should

show a condensed diff of the entire history. This will be addressed in a

later patch.
This also allows the original and patched file API endpoints to work for

files modified multiple times in a commit history.

Ran unit tests.
Was able to view diffs in the diffviewer that depended on previous

  FileDiffs.