Provide maximum comparison length onboard PhutilEditDistanceMatrix

Summary: After some maximum length, we bail out rather than compute the acutal edit distance because the time and memory complexity grows nonlinearly. Bring this check onboard PhutilEditDistanceMatrix.

Test Plan: Added and ran unit tests.

Reviewers: btrahan, Afaque_Hussain, ivane2004

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2379

Differential Revision: https://secure.phabricator.com/D6015