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

@@ -74,7 +74,7 @@ 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
@@ -85,7 +85,7 @@ 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

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
     ;;