Move Ggit.Repository.file_status() call into a worker thread

Otherwise large repositories can block the gedit UI.
Split long line
Fix monitoring for file event

Was checking if the Gio.File was in a set which contains URIs.
Small cleanup
There might not be a repository on inserted()
Clear files and file_names when the file browser's root changes
Require a python3 interpreter in configure.ac

Otherwise byte-compiling python modules will fail when
new python3 syntax is used.
Add debug messages to git plugin