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

@@ -55,7 +55,7 @@ TODO: 3 of 3 tasks shown
 
 >>> 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
@@ -66,7 +66,7 @@ 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

@@ -1091,18 +1091,23 @@ 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
     ;;