Fix default for syntax_aliases not being respected

kittens/diff/__init__.py

@@ -2,4 +2,8 @@ from typing import Dict
 
 
 def syntax_aliases(x: str) -> Dict[str, str]:
-    return {}
+    ans = {}
+    for x in x.split():
+        k, _, v = x.partition(':')
+        ans[k] = v
+    return ans