Only run Black on modified files

This restores the intelligent file selection that the previous formatter
was set up to use.

Author: Zalathar

tooling/src/hypothesistooling/__main__.py

@@ -216,7 +216,7 @@ def should_format_file(path):
             o.write("\n")
     pip_tool("isort", *files_to_format)
 
-    pip_tool("black", tools.ROOT)
+    pip_tool("black", *files_to_format)
 
 
 VALID_STARTS = ("# coding=utf-8", "#!/usr/bin/env python")