Extract and cover the logic for "trimming" a lint message

Summary:
Ref T9846. Sometimes, a lint message says to replace "the big bad wolf" with "the huge bad wolf": that is, the original and replacement text are the same at the beginning, or the end, or both.

To make this easier for humans to understand, we want to just show that "big" is being replaced with "huge", not that the entire phrase is being replaced.

This logic currently happens inline in console rendering. Pull it out and cover it so a future change can simplify console rendering.

Test Plan: Ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9846

Differential Revision: https://secure.phabricator.com/D18509