Display compatible inline comments (usually synthetic) on the same row

Summary:
This just looks silly:
{F8088, size=full}

It runs in O(N*N) but it's not a big deal because there are usually only few
comments per line.
I didn't implement it for images.

Test Plan:
View revision with compatible inline comments in two diffs.
View revision with different inline comments on same line in two diffs.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1570