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

src/workflow/ArcanistDiffWorkflow.php

@@ -997,6 +997,10 @@ EOTEXT
           if ($raw_change->getCurrentPath() == $path) {
             $change->setFileType($raw_change->getFileType());
             foreach ($raw_change->getHunks() as $hunk) {
+              // Git thinks that this file has been added. But we know that it
+              // has been moved or copied without a change.
+              $hunk->setCorpus(
+                preg_replace('/^\+/m', ' ', $hunk->getCorpus()));
               $change->addHunk($hunk);
             }
             break;