DiffXParser stores parsed information on a DiffX file in DiffSet,

DiffCommit, and FileDiff, and then uses that information to

regenerate the DiffX file.
The problem is, if a diff is uploaded without commit history (due to API

usage or SCM limitation), there is no DiffCommit, so we were losing

out on all the change-related data and all associated files. This isn't

just an issue for DiffX, but any parser storing data in

ParsedDiffChange.extra_data.
FileDiff creation now checks for this situation and stores any

ParsedDiffChange.extra_data content in a special key in DiffSet

(change_extra_data for a main diff, parent_change_extra_data for a

parent diff). DiffXParser will look for this and use it if commits

aren't available.

Unit tests pass on Python 2 and 3.
Verified that this fixed issues in production with non-commit-backed

diffs.