mergemaster: Clarify installed and updated versions

Describe "diff installed new" as "Displaying differences between
installed and new."  Previously mergemaster described them in the
opposite order.

PR:		249214
Reported by:	Yuri Victorovich
MFC after:	2 weeks

usr.sbin/mergemaster/mergemaster.sh

@@ -143,7 +143,7 @@ diff_loop () {
 	echo '   ======================================================================   '
 	echo ''
         (
-          echo "  *** Displaying differences between ${COMPFILE} and installed version:"
+          echo "  *** Displaying differences between installed version and ${COMPFILE}:"
           echo ''
           diff ${DIFF_FLAG} ${DIFF_OPTIONS} "${DESTDIR}${COMPFILE#.}" "${COMPFILE}"
         ) | ${PAGER}