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)

tests/t1200-pri.sh

@@ -53,9 +53,28 @@ TODO: 2 prioritized (C).
 --
 TODO: 3 of 3 tasks shown
 
+>>> todo.sh add "smell the coffee +wakeup"
+4 smell the coffee +wakeup
+TODO: 4 added.
+
+>>> todo.sh -p list
+1 (B) smell the uppercase Roses +flowers @outside
+2 (C) notice the sunflowers
+4 smell the coffee +wakeup
+3 stop
+--
+TODO: 4 of 4 tasks shown
+EOF
+
+cat > todo.txt <<EOF
+(B) smell the uppercase Roses +flowers @outside
+(C) notice the sunflowers
+stop
+EOF
+test_todo_session 'reprioritize' <<EOF
 >>> todo.sh pri 2 A
 2 (A) notice the sunflowers
-TODO: 2 prioritized (A).
+TODO: 2 re-prioritized from (C) to (A).
 
 >>> todo.sh -p list
 2 (A) notice the sunflowers
@@ -66,25 +85,12 @@ TODO: 3 of 3 tasks shown
 
 >>> todo.sh pri 2 a
 2 (A) notice the sunflowers
-TODO: 2 prioritized (A).
+TODO: 2 already prioritized (A).
 
 >>> todo.sh -p listpri
 2 (A) notice the sunflowers
 1 (B) smell the uppercase Roses +flowers @outside
 --
 TODO: 2 of 3 tasks shown
-
->>> todo.sh add "smell the coffee +wakeup"
-4 smell the coffee +wakeup
-TODO: 4 added.
-
->>> todo.sh -p list
-2 (A) notice the sunflowers
-1 (B) smell the uppercase Roses +flowers @outside
-4 smell the coffee +wakeup
-3 stop
---
-TODO: 4 of 4 tasks shown
 EOF
-
 test_done

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"
+    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
-    else
-        die "$errmsg"
+        fi
+    fi
+    if [ "$oldpri" = "$newpri" ]; then
+        echo "TODO: $item already prioritized ($newpri)."
     fi
     ;;