Two FileDiffs are equivalent if and only if they have the same diff

content (including headers). Instead of directly comparing the diff

contents we can instead see that they point at the same

RawFileDiffData object, as any two identical FileDiffs must have

identical RawFileDiffData instances.
Two DiffCommits are equivalent if and only if they have the same

author, committer, and diff contents. That is, every FileDiff in

DiffCommit A will have a corresponding equivalent Filediff in

DiffCommit B if A and B are equivalent. We cache the result of this

comparison in the extra_data of each DiffCommit to save future

database queries.

Ran unit tests.