Implementing prefix and suffix diffing for multi-byte unicode characters.

Summary: Prefix and suffix diffing for multi-byte unicode characters.

Test Plan:
Test diff:
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diff --git a/test b/test
index 252ad2f..a5bad3b 100644
--- a/test
+++ b/test
@@ -1 +1 @@
-This file contains this and this stuff.
+This file contains žžžž and this stuff.
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{F43738}
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Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T2379

Differential Revision: https://secure.phabricator.com/D5925