pri: Check for existing priority and adapt message.

The existing check (as with depri) didn't do much good. Instead, fetch the existing priority and use that information to print more specific messages:
- TODO: 42 re-prioritized from (C) to (A)
- TODO: 42 already prioritized (A)

todo.sh

@@ -1086,18 +1086,23 @@ note: PRIORITY must be anywhere from A to Z."
     [[ "$item" = +([0-9]) ]] || die "$errmsg"
     [[ "$newpri" = @([A-Z]) ]] || die "$errmsg"
 
-    sed -e $item"s/^(.) //" -e $item"s/^/($newpri) /" "$TODO_FILE" > /dev/null 2>&1
-
-    if [ "$?" -eq 0 ]; then
-        #it's all good, continue
+    oldpri=$(sed -ne $item's/^(\(.\)) .*/\1/p' "$TODO_FILE")
+    if [ "$oldpri" != "$newpri" ]; then
         sed -i.bak -e $item"s/^(.) //" -e $item"s/^/($newpri) /" "$TODO_FILE"
-        if [ $TODOTXT_VERBOSE -gt 0 ]; then
-            NEWTODO=$(sed "$item!d" "$TODO_FILE")
-            echo "$item $NEWTODO"
-            echo "TODO: $item prioritized ($newpri)."
-	fi
-    else
-        die "$errmsg"
+    fi
+    if [ $TODOTXT_VERBOSE -gt 0 ]; then
+        NEWTODO=$(sed "$item!d" "$TODO_FILE")
+        echo "$item $NEWTODO"
+        if [ "$oldpri" != "$newpri" ]; then
+            if [ "$oldpri" ]; then
+                echo "TODO: $item re-prioritized from ($oldpri) to ($newpri)."
+            else
+                echo "TODO: $item prioritized ($newpri)."
+            fi
+        fi
+    fi
+    if [ "$oldpri" = "$newpri" ]; then
+        echo "TODO: $item already prioritized ($newpri)."
     fi
     ;;