core: fix Python 3 support on Windows

Replace os.getcwdu() call in start_command() with getcwd() from
cola.core.

Signed-off-by: Daniel Harding <dharding@living180.net>
widgets.status: fix copy_path() on Python 3

Replace call to os.getcwdu() with core.getcwd().

Signed-off-by: Daniel Harding <dharding@living180.net>