common/hooks: add support for vcdiff creation

common/hooks/pre-pkg/01-xdelta_repolist.sh

@@ -0,0 +1,11 @@
+# this hook marks files which are about to change for generating vcdiffs
+
+hook() {
+	[ -z "$XBPS_GENERATE_VCDIFF" ] && return 0;
+
+	# create links to preserve old versions of repodata
+	find $XBPS_REPOSITORY -name '*-repodata' | \
+		while read; do
+			ln "${REPLY}" "${REPLY}.genVcdiff"
+		done
+}