Check for fixup commits on CI

I keep merging PRs that still have fixup commits on them! This will make
it impossible to do so

scripts/check_for_fixups.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+base_ref=$1
+
+# Determine the base commit
+base_commit=$(git merge-base HEAD origin/"$base_ref")
+
+# Check if base_commit is set correctly
+if [ -z "$base_commit" ]; then
+    echo "Failed to determine base commit."
+    exit 1
+fi
+echo "Base commit: $base_commit"
+
+# Get commits with "fixup!" in the message from base_commit to HEAD
+commits=$(git log -i -P --grep "fixup\!" --format="%h %s" "$base_commit..HEAD")
+
+if [ -z "$commits" ]; then
+    echo "No fixup commits found."
+    exit 0
+else
+    echo "Fixup commits found:"
+    echo "$commits"
+    exit 1
+fi