ENH: 'prepend' and 'replace' actions keep prepended date.

Generalized and simplified the logic that already kept an existing priority to also keep a date added via todo.sh -t / TODOTXT_DATE_ON_ADD (unless the replaced text also starts with a date).

tests/t1400-prepend.sh

@@ -57,4 +57,38 @@ test_todo_session 'prepend with &' <<EOF
 3: no running & jumping now stop
 EOF
 
+cat /dev/null > todo.txt
+test_todo_session 'prepend handling prepended date on add' <<EOF
+>>> todo.sh -t add "new task"
+1: 2009-02-13 new task
+TODO: 1 added.
+
+>>> todo.sh prepend 1 "this is just a"
+1: 2009-02-13 this is just a new task
+EOF
+
+cat /dev/null > todo.txt
+test_todo_session 'prepend handling priority and prepended date on add' <<EOF
+>>> todo.sh -t add "new task"
+1: 2009-02-13 new task
+TODO: 1 added.
+
+>>> todo.sh pri 1 A
+1: (A) 2009-02-13 new task
+TODO: 1 prioritized (A).
+
+>>> todo.sh prepend 1 "this is just a"
+1: (A) 2009-02-13 this is just a new task
+EOF
+
+cat /dev/null > todo.txt
+test_todo_session 'prepend with prepended date keeps both' <<EOF
+>>> todo.sh -t add "new task"
+1: 2009-02-13 new task
+TODO: 1 added.
+
+>>> todo.sh prepend 1 "2010-07-04 this is just a"
+1: 2009-02-13 2010-07-04 this is just a new task
+EOF
+
 test_done