Refactoring: Extract git checks in push and merge scripts.

This extracts the pattern "if call git fails: raise exception", which is spread all over the place. Now all calls to git are required to return gracefully and give a uniform exception message if they don't.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/166903012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19494 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/bleeding_edge' into bleeding_edge