Use git worktree in diffcheck.sh to avoid mutating the working tree

scripts/diffcheck.sh

@@ -1,15 +1,47 @@
 #!/bin/bash
 
-scripts/buildtable.pl >/tmp/table.mediawiki 2> /dev/null
-diff README.mediawiki /tmp/table.mediawiki | grep '^[<>] |' >/tmp/after.diff || true
-if git checkout HEAD^ && scripts/buildtable.pl >/tmp/table.mediawiki 2>/dev/null; then
-    diff README.mediawiki /tmp/table.mediawiki | grep '^[<>] |' >/tmp/before.diff || true
-    newdiff=$(diff -s /tmp/before.diff /tmp/after.diff -u | grep '^+')
-    if [ -n "$newdiff" ]; then
-        echo "$newdiff"
-        exit 1
-    fi
-    echo "README table matches expected table from BIP files"
+set -Eeuo pipefail
+
+after_diff="$(mktemp)"
+before_diff="$(mktemp)"
+table_current="$(mktemp)"
+table_prev="$(mktemp)"
+
+# Ensure previous commit is available (CI often uses shallow clones)
+if ! git rev-parse --verify -q HEAD^ >/dev/null; then
+	git fetch --deepen=100 >/dev/null 2>&1 || git fetch --unshallow >/dev/null 2>&1 || true
+fi
+
+# If previous commit is still unavailable, skip diff check gracefully
+if ! git rev-parse --verify -q HEAD^ >/dev/null; then
+	echo "Skipping diff check: previous commit (HEAD^) is unavailable in this environment"
+	exit 0
+fi
+
+tmpdir="$(mktemp -d)"
+cleanup() {
+	git worktree remove --force "$tmpdir" >/dev/null 2>&1 || true
+	rm -f "$table_current" "$table_prev" "$before_diff" "$after_diff" >/dev/null 2>&1 || true
+}
+trap cleanup EXIT
+
+# Build table from current workspace
+scripts/buildtable.pl >"$table_current" 2> /dev/null
+diff README.mediawiki "$table_current" | grep '^[<>] |' >"$after_diff" || true
+
+# Build table from previous commit using a detached worktree
+if git worktree add --detach "$tmpdir" HEAD^ >/dev/null 2>&1; then
+	(
+		cd "$tmpdir"
+		scripts/buildtable.pl >"$table_prev" 2>/dev/null
+		diff README.mediawiki "$table_prev" | grep '^[<>] |' >"$before_diff" || true
+	)
+	newdiff=$(diff -s "$before_diff" "$after_diff" -u | grep '^\+')
+	if [ -n "$newdiff" ]; then
+		echo "$newdiff"
+		exit 1
+	fi
+	echo "README table matches expected table from BIP files"
 else
     echo 'Cannot build previous commit table for comparison'
     exit 1