do not measure coverage for fuse.py

it's always at 0% although we do have fuse tests. guess one can't measure such code with coverage.py.

.coveragerc

@@ -5,6 +5,7 @@ omit =
     */borg/__init__.py
     */borg/__main__.py
     */borg/_version.py
+    */borg/fuse.py
     */borg/support/*
     */borg/testsuite/*