When inlines would disable a file shield in a diff, still apply the shield if all the comments are collapsed

Summary:
Ref T13515. We "shield" some changesets, including generated code and intradiffs with no intermediate changes.

These files don't get shielded if they have inline comments.

But, if the viewer has collapsed all the comments, we can shield the file again.

Test Plan:

• Created a change affecting files A and B, with three diffs:
  • Touch A and B.
  • Touch B only.
  • Touch nothing.
• Added an inline to A and collapsed it.
• Viewed Diff 1 vs Diff 2:
  • Saw A collapse with a note about inlines.
  • Saw B changes, normally.
• Viewed Diff 2 vs Diff 3:
  • Saw A collapse with a note about inlines.
  • Saw B collapse normally.
• Uncollapsed the inline, viewed 1v2 and 2v3, saw A expand in both cases.

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21136