Assume unknown verbosity == INFO

Not WARNING, I think INFO is a much safer choice. And for now, it's
equivalent to WARNING, since we use only DEBUG and INFO.

borgmatic/tests/unit/test_verbosity.py

@@ -8,4 +8,4 @@ def test_verbosity_to_log_level_maps_known_verbosity_to_log_level():
 
 
 def test_verbosity_to_log_level_maps_unknown_verbosity_to_warning_level():
-    assert module.verbosity_to_log_level('my pants') == logging.WARNING
+    assert module.verbosity_to_log_level('my pants') == logging.INFO