Refactoring: Extract low-level git from push and merge scripts.

Refactoring: Extract low-level git from push and merge scripts.

- This adds a python layer on top of the low-level git calls to control parameter passing and return values.
- This also fixes a bug in FindLastTrunkPush that only shows up in manual mode when the last push versions are iterated.
- The order of some parameters changed in some git calls in the tests to be uniform.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/171423013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19515 ce2b1a6d-e550-0410-aec6-3dcde31c8c00