git: fix command windows popping up on Windows

Prior to commit 1109aeb4, the `Git.execute` method passed `shell=True` to `core.run_command` on Windows systems.  That commit stopped passing the `shell` parameter to `core.run_command`, with the result that every time `Git.execute` was called on Windows, a command shell window would flash on the screen.  This commit restores the pre- 1109aeb4 behavior.

Tested on Windows 7 using Python 2.7.6 and Python 3.3.5.