Tested with:
* Unit tests have been added and they work.
* I also tested end-to-end using a skiabot-test repository in https://skia.googlesource.com/ (it is hidden). CLs I tested with are:
** https://codereview.chromium.org/22797006/ : Add whitespace in file1
** https://codereview.chromium.org/22815013/ : Remove whitespace from file1
** https://codereview.chromium.org/22867025/ : Add new files in directories
** https://codereview.chromium.org/22901018/ : Edit file in directory and delete file in directory
** https://codereview.chromium.org/22918014/ : Add, Delete and Modify 3 files
** https://codereview.chromium.org/23360004/ : Add new files in new directories


Note:
* When committing GitCheckout uses the --author to specify the original author. The author flag takes in 'Firstname Lastname <email_addr>' but we do not know the Firstname and LastName of the original author, which is why the code here parses out the username from the email address and uses it.
  Eg: For email address xyz@example.com it passes in --author 'xyz <xyz@example.com>'
* An example of the changes required in a project to use GitCheckout instead of SvnCheckout is in https://codereview.chromium.org/22859063/


Created to fix the following feature requests-
https://code.google.com/p/chromium/issues/detail?id=261619 : Update the Chrome commit queue to push to src.git.
https://code.google.com/p/skia/issues/detail?id=1593 : Add Git support to the Commit Queue.

Review URL: https://chromiumcodereview.appspot.com/22794015

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@221392 0039d316-1c4b-4281-b951-d872f2087c98