preserve the previous log too

releng/run_all_releng

@@ -28,6 +28,9 @@ HLP
 fi
 
 ########################################################
+ if [ -f "${CLOG}" ]; then
+   mv "${CLOG}" "${CLOG}".prev
+ fi
  > ${CLOG}
  DSTART=`date --utc`
  printf "\n${SELF}: Started at ${DSTART}"
@@ -66,6 +69,13 @@ fi
    let ES="${E2} - ${E1}"
    printf "\n${SELF}: Total time elapsed ${ES} sec."
    printf "\n${SELF}: stdout&stderr saved in ${CLOG}.\n"
+
+# TODO: work out a less noisy diff'ing technique!
+#   if [ -f "${CLOG}".prev -a -f "${CLOG}" ]; then
+#     printf "\n${SELF}: Log diff follows:\n"
+#     diff -Naur "${CLOG}".prev "${CLOG}"
+#   fi
+
  fi
 
  exit 0