Build correct corpus in copied or moved files

Summary:
This problem shows very far away.
One of the symptomps is that the contents of a moved file is displayed as added in Differential but it is not a big deal.

The real trouble happens when you try to arc patch this diff.
It tries to both copy the file and to add a new contents (which fails).

Fixes T1709.

Test Plan:

$ git mv a b
$ git commit -m.
$ arc diff --only
$ git reset --hard HEAD^
$ arc patch --diff # of the created diff

$ arc unit src/parser/__tests__

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: aran, Korvin, boris, mroch, slawekbiel

Maniphest Tasks: T1709

Differential Revision: https://secure.phabricator.com/D3524