Let relocate photo accept the old filepath in place of photo id.

frontends/etiquette_cli.py

@@ -497,6 +497,9 @@ def relocate_argparse(args):
     load_photodb()
 
     with photodb.transaction:
+        if '.' in args.photo_id:
+            photo = photodb.get_photo_by_path(args.photo_id)
+        else:
             photo = photodb.get_photo(args.photo_id)
         photo.relocate(args.filepath)
 
@@ -1310,6 +1313,9 @@ def main(argv):
     )
     p_relocate.add_argument(
         'filepath',
+        help='''
+        The new filepath of the photo.
+        ''',
     )
     p_relocate.add_argument(
         '--yes',