This makes some important changes to how we store information on source

files referenced in parent diffs, accompanying information for the

primary diff, and how we look up files.
When parent diffs were added, we had some restrictions on our schema.

There was no extra_data, and we only really had the source/destination

file/revision fields to work with. At that point, we opted to set the

source file/revision fields to point to the source of the parent diff,

so that we would fetch the right source file to apply the patch to.
This wasn't really ideal, as it sort of masked the actual revisions of

the modified file the reviewer was looking at. It was also broken -- we

didn't actually update the source filename, just the revision.
That latter issue is what led to this change in the first place. On Git

(which we first introduced parent diff support for), we didn't need that

filename, just the revision (the blob SHA), so a renamed parent file

didn't break anything.
In Mercurial, though, we need both the file and revision. Not having the

true original filename resulted in breakages when the file was

moved/renamed in the parent diff.
Going forward, we store the parent source filename/revision in

FileDiff.extra_data, and we keep the FileDiff fields specific to the

primary diff posted for review.
get_original_file() will attempt to locate using the parent source, if

available, and fall back on the main FileDiff source. This keeps

everything working as it always has for any existing diffs (and of

course for diffs without a parent diff), but allows us to maintain this

clean separation going forward.
Anyone with diffs in a database that broke due to a moved/renamed parent

source filename, who had other means of working around it, can simply

update the FileDiff.extra_data keys to point to the correct location,

fixing the issue without having to re-upload diffs.

Unit tests pass.
Tested viewing existing diffs containing parent diffs, and didn't hit any

issues.
Posted new Mercurial and Git diffs with parent diffs, both with and

without files being moved. In all cases, the source file were correctly

fetched and the files rendered with the expected information.