Enable copy detection for git-cl upload.  This makes
it possible to copy a directory containing many files,
add+commit the new path, and then upload a patch to
Rietveld that shows the files were copied.

In my tests, -C -C was needed to pick up a basic
file copy.  -C was not enough.  I'm not sure why
exactly.  The output generated in the diff looks
like:

  sh$ git diff -C -C HEAD~1..HEAD
  diff --git a/slave/Makefile b/slave2/Makefile
  similarity index 100%
  copy from slave/Makefile
  copy to slave2/Makefile
  ...

Review URL: https://chromiumcodereview.appspot.com/10412027

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@140314 0039d316-1c4b-4281-b951-d872f2087c98