Summary:

Rename `diff_set` -> `diffset`, etc.

Review Request #9994 — Created May 31, 2018 and submitted July 10, 2018, 7:55 p.m. — Latest diff uploaded July 4, 2018, 3:04 p.m.

Information
Owner:	brennie
Repository:	Review Board
Branch:	release-4.0.x
Bugs:
Depends On:
Blocks:	9609
Commit:	0067603...
Reviewers
Groups:	reviewboard
People:

Description

This patch renames the model files for DiffSet, FileDiff, and
DiffSetHistory to treat Diff{...} as a single word in the filenames.
All docstrings have been udpated to refer to the new location.

Testing Done

Ran unit tests.