FIX: Adapt deduplicate sed command for OS X.

The sed command of BSD / OS X doesn't like { command } blocks in a single expression. So move the (dynamic) deduplicate sed command(s) to the end, and use a label to access them. This also makes the entire sed script somewhat easier to understand.

tests/t1910-deduplicate.sh

@@ -82,9 +82,9 @@ EOF
 
 cat > todo.txt <<EOF
 normal task
-a bold action
+a bold task
 something else
-a bold action
+a bold task
 something more
 EOF
 test_todo_session 'deduplicate with non-printable duplicates' <<EOF
@@ -92,7 +92,7 @@ test_todo_session 'deduplicate with non-printable duplicates' <<EOF
 TODO: 1 duplicate task(s) removed
 
 >>> todo.sh -p ls
-2 a bold action
+2 a bold task
 1 normal task
 3 something else
 5 something more