Replace git diff-tree invocation

Gerrit currently runs git diff-tree to discover changed files.
That command might not be available in all environments, so I
replaced it with a Java based implementation.

The Java based implementation does not yet support whitespace
ignore, rename detection, or merge commits.

Change-Id: I0adfb7b68e7beba635176f1295c442ffdd397d4e
(cherry picked from commit 34f5af4c854da484db05f6570f5022761858b1be)
Change-Id: Ib76aaad601c858399a58219fdf179a55bdaacf85