git: Add post-rewrite hook that invokes git submodule update

git rebase does not support a --recurse-submodules switch to automatically
check out the submodules at their registered commits during or after a rebase.

Instead, let's use the post-rewrite git hook to do this ourselves.

docs/HACKING.md

@@ -21,6 +21,8 @@ git correctly (running `meson` will run these commands for you automatically):
 $ git config submodule.recurse true
 $ git config fetch.recurseSubmodules on-demand
 $ git config push.recurseSubmodules no
+$ cp .git/hooks/pre-commit.sample .git/hooks/pre-commit
+$ cp tools/git-post-rewrite-hook.sh .git/hooks/post-rewrite
 ```
 
 When adding new functionality, tests should be added. For shared functionality

tools/git-post-rewrite-hook.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+exec git submodule update

tools/git-setup.sh

@@ -10,10 +10,19 @@ if [ -e .git ]; then
     git config push.recurseSubmodules no
 fi
 
-if [ ! -f .git/hooks/pre-commit.sample ] || [ -f .git/hooks/pre-commit ]; then
-    exit 2 # not needed
+ret=2
+
+if [ -f .git/hooks/pre-commit.sample ] && [ ! -f .git/hooks/pre-commit ]; then
+    cp -p .git/hooks/pre-commit.sample .git/hooks/pre-commit
+    chmod +x .git/hooks/pre-commit
+    echo 'Activated pre-commit hook'
+    ret=0
 fi
 
-cp -p .git/hooks/pre-commit.sample .git/hooks/pre-commit
-chmod +x .git/hooks/pre-commit
-echo 'Activated pre-commit hook'
+if [ ! -f .git/hooks/post-rewrite ]; then
+    cp -p tools/git-post-rewrite-hook.sh .git/hooks/post-rewrite
+    echo 'Activated post-rewrite hook'
+    ret=0
+fi
+
+exit $ret