When showing a diff-of-diffs, hide files which didn't get any more changes and have no inlines

Summary:
Ref T13137. See that task for discussion.

When we show a diff-of-diffs, we often render stubs for files which didn't change between the diffs. These stubs usually aren't a big deal, but for certain types of changes (like refactors) they can create a lot of clutter.

Instead, hide these stubs and show a notice that we hid them.

Test Plan:

• Created a revision affecting 4 files.
• Updated it with a diff that changed only 1 of the 4 files.
• Added an inline comment to a different file.
• Viewed the diff of diffs.
  • Before: 4 changesets with two "nothing changed" stubs.
  • After: 2 changesets with the stubs hidden.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13137

Differential Revision: https://secure.phabricator.com/D19453