Added post-rewrite hook to automatically update submodules when

they change.

git-hooks/README.md

@@ -0,0 +1,4 @@
+This directory contains git hooks (programs automatically triggered by git events).
+They cannot be put in ./git/hooks by way of version control; you have to copy them
+there yourself. Make sure they are executable.
+

git-hooks/post-rewrite

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+#!/bin/bash
+# from https://gist.github.com/digitaljhelms/f74eaf56835262d6bf3f
+echo "[post rewrite hook: $1]"
+
+# quick script to call "git submodule update" automatically if the
+# .gitmodules file is changed
+
+changedfiles=( `git diff-tree --no-commit-id --name-only HEAD@{1} HEAD` )
+
+if [[ "${changedfiles[*]}" =~ ".gitmodules" ]]; then
+    echo "initializing & updating submodule(s)"
+    git submodule update --init --recursive
+fi