Fix minor inline comment header button behaviors

Summary:
Fixes T12806. Ref T12733.

• Don't count synthetic (lint) comments as anything.
• When you begin writing an inline then cancel it, don't count it as anything.
• When we would show "0 / X", just show "X".

Test Plan:

• Viewed a diff with synthetic comments, no button.
• Wrote, then cancelled an inline. No "X comments".
• Clicked / unlicked "Done", saw "X" -> "1 / X".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12806, T12733

Differential Revision: https://secure.phabricator.com/D18103