Fixing phutil_utf8_shorten().
Summary: Fixed phutil_utf8_shorten() to be combining character aware.
Test Plan: Unit tests. However, I think I may have not added enough test cases.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin, AnhNhan
Maniphest Tasks: T2379
Differential Revision: https://secure.phabricator.com/D6025