isort: print diff on failures

This should help users figure out how to fix problems.

Change-Id: I6346e163d06f9a0e52147689c5b329fe7f1366d2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/635343
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

run_tests

@@ -135,7 +135,7 @@ def run_isort():
         cwd=ROOT_DIR,
     )
 
-    argv = ["--check", ROOT_DIR]
+    argv = ["--check", "--diff", ROOT_DIR]
     log_cmd("isort", argv)
     return subprocess.run(
         [sys.executable, "-m", "isort"] + argv,