Break out the pre-commit hook into an explicit script

This means that

- we don't have to be always updating the underlying .git/hooks/pre-commit file
- updates to the logic will just work automatically
- the logic of the black logic has been fixed so that AWX_IGNORE_BLACK=1 should work right now

pre-commit.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/bash
+
+if [ -z $AWX_IGNORE_BLACK ]
+then
+        black --check $(git diff --cached --name-only --diff-filter=AM | grep -E '\.py') || \
+        (echo 'To fix this, run `make black` to auto-format your code prior to commit, or set AWX_IGNORE_BLACK=1' && \
+        exit 1)
+fi