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.