Feature request - add a flag to specify the path to google-java-format-diff.py

Context: https://android-review.googlesource.com/#/c/314751

We're having to resort to editing $PATH before calling google-java-format-diff.py because the tool doesn't support specifying the path manually. This is a bit hacky - it would be nice if the script supported taking the path to the binary directly before resorting to find_executable or the hard-coded path in /usr/bin.