ENH: 'prepend' and 'replace' actions keep prepended date.

Generalized and simplified the logic that already kept an existing priority to also keep a date added via todo.sh -t / TODOTXT_DATE_ON_ADD (unless the replaced text also starts with a date).

todo.sh

@@ -288,8 +288,14 @@ replaceOrPrepend()
 {
   action=$1; shift
   case "$action" in
-    replace) backref=;;
-    prepend) backref=' &';;
+    replace)
+      backref=
+      querytext="Replacement: "
+      ;;
+    prepend)
+      backref=' &'
+      querytext="Prepend: "
+      ;;
   esac
   shift; item=$1; shift
 
@@ -300,27 +306,27 @@ replaceOrPrepend()
   [ -z "$todo" ] && die "$item: No such task."
 
   if [[ -z "$1" && $TODOTXT_FORCE = 0 ]]; then
-    case "$action" in
-      replace) echo -n "Replacement: ";;
-      prepend) echo -n "Prepend: ";;
-    esac
+    echo -n "$querytext"
     read input
   else
     input=$*
   fi
   cleaninput $input
 
-  # Test for then set priority
-  if [ `sed "$item!d" "$TODO_FILE"|grep -c "^(\\w)"` -eq 1 ]; then
-    priority=$(sed "$item!d" "$TODO_FILE" | awk -F '\\(|\\)' '{print $2}')
+  # Retrieve existing priority and prepended date
+  priority=$(sed -e "$item!d" -e $item's/^\(([A-Z]) \)\{0,1\}\([0-9]\{2,4\}-[0-9]\{2\}-[0-9]\{2\} \)\{0,1\}.*/\1/' "$TODO_FILE")
+  prepdate=$(sed -e "$item!d" -e $item's/^\(([A-Z]) \)\{0,1\}\([0-9]\{2,4\}-[0-9]\{2\}-[0-9]\{2\} \)\{0,1\}.*/\2/' "$TODO_FILE")
+
+  if [ "$prepdate" -a "$action" = "replace" ] && [ "$(echo "$input"|sed -e 's/^\([0-9]\{2,4\}-[0-9]\{2\}-[0-9]\{2\}\)\{0,1\}.*/\1/')" ]; then
+    # If the replaced text starts with a date, it will replace the existing
+    # date, too.
+    prepdate=
   fi
 
-  # If priority isn't set change task, if it is remove priority, change then add priority again
-  if [ -z $priority ]; then
-    sed -i.bak $item" s|^.*|${input}${backref}|" "$TODO_FILE"
-  else
-    sed -i.bak -e "$item s/^(.) //" -e "$item s|^.*|\($priority\) ${input}${backref}|" "$TODO_FILE"
-  fi
+  # Temporarily remove any existing priority and prepended date, perform the
+  # change (replace/prepend) and re-insert the existing priority and prepended
+  # date again.
+  sed -i.bak -e "$item s/^${priority}${prepdate}//" -e "$item s|^.*|${priority}${prepdate}${input}${backref}|" "$TODO_FILE"
   if [ $TODOTXT_VERBOSE -gt 0 ]; then
     newtodo=$(sed "$item!d" "$TODO_FILE")
     case "$action" in