Fix display of diffs of files that were renamed in a parent diff

In order for this to work it was necessary to add a new column to the FileDiff
table: parent_file. This stores the name of the file in the base revision,
which may be different from the names of the file in either or both of the
source and destination revisions.

- Posted several diffs with parent diffs that rename files. Scenarios:
1. Rename in parent diff, no rename in review diff
2. Rename in review diff, no rename in parent diff
3. Rename in both parent diff and review diff

- Added unit tests for looking up files with the key scenarios that were failing before.