Fix infinite loop in PhutilFileTree

Summary: In one of our diff we have files with name '10.8' and '10.80' in the same dir, which caused infinite loop since numerically they are the same hence getNextSibling() call on 10.80 will always gets itself.

Test Plan: Load the diff with fix.

Reviewers: epriestley, lifeihuang, JoelB, cdentel, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8091