Fix a prefix/suffix counting issue in Arcanist lint rendering

Summary: Ref T9846. See PHI48. For replacing text in the form "ABC" with "ABBC", the trimmer had a bug.

Test Plan: Added failing tests, fixed 'em.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9846

Differential Revision: https://secure.phabricator.com/D18538