Support "preserve line numbers" in deduplicate.

tests/t1910-deduplicate.sh

@@ -16,10 +16,38 @@ double task
 three
 EOF
 
-test_todo_session 'deduplicate with duplicates' <<EOF
+test_todo_session 'deduplicate and preserve line numbers' <<EOF
 >>> todo.sh deduplicate
 TODO: 2 duplicate task(s) removed
 
+>>> todo.sh -p ls
+5 double task
+1 duplicated
+7 three
+2 two
+3 x done
+--
+TODO: 5 of 5 tasks shown
+EOF
+
+test_todo_session 'deduplicate without duplicates' <<EOF
+>>> todo.sh deduplicate
+TODO: No duplicate tasks found
+EOF
+
+cat > todo.txt <<EOF
+duplicated
+two
+x done
+duplicated
+double task
+double task
+three
+EOF
+test_todo_session 'deduplicate and delete lines' <<EOF
+>>> todo.sh -n deduplicate
+TODO: 2 duplicate task(s) removed
+
 >>> todo.sh -p ls
 4 double task
 1 duplicated
@@ -30,9 +58,26 @@ TODO: 2 duplicate task(s) removed
 TODO: 5 of 5 tasks shown
 EOF
 
-test_todo_session 'deduplicate without duplicates' <<EOF
+cat > todo.txt <<EOF
+one
+duplicated
+three
+duplicated
+duplicated
+six
+duplicated
+EOF
+test_todo_session 'deduplicate more than two occurrences' <<EOF
 >>> todo.sh deduplicate
-TODO: No duplicate tasks found
+TODO: 3 duplicate task(s) removed
+
+>>> todo.sh -p ls
+2 duplicated
+1 one
+6 six
+3 three
+--
+TODO: 4 of 4 tasks shown
 EOF
 
 cat > todo.txt <<EOF
@@ -50,7 +95,7 @@ TODO: 1 duplicate task(s) removed
 2 a bold action
 1 normal task
 3 something else
-4 something more
+5 something more
 --
 TODO: 4 of 4 tasks shown
 EOF