Provide onboard optimizations for strings with similar prefixes and suffixes in PhutilEditDistanceMatrix

Summary: We can evaluate similar prefixes and suffixes in O(1). Provide this optimization within PhutilEditDistanceMatrix.

Test Plan: Ran unit tests.

Reviewers: btrahan, Afaque_Hussain

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2379

Differential Revision: https://secure.phabricator.com/D6014