Fix SnapshotProvider failing on taking_snapshot without changes

lib/tfw/components/snapshot_provider.py

@@ -66,10 +66,14 @@ class SnapshotProvider:
             'git', 'add',
             '-A'
         ))
-        self._run((
+        try:
+            self._get_stdout((
                 'git', 'commit',
                 '-m', 'Snapshot'
             ))
+        except CalledProcessError as err:
+            if b'nothing to commit, working tree clean' not in err.output:
+                raise
 
     def _check_head_not_detached(self):
         if self._head_detached: