filter-repo: refine choice of when to skip blobs

We can pass --no-data to fast-export in one additional case.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -3206,8 +3206,7 @@ class RepoFilter(object):
     else:
       skip_blobs = (self._blob_callback is None and
                     self._args.replace_text is None and
-                    self._args.source is None and
-                    self._args.target is None)
+                    self._args.source == self._args.target)
       extra_flags = []
       if skip_blobs:
         extra_flags.append('--no-data')