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/t1100-replace.sh

@@ -77,6 +77,7 @@ replaced with
 replaced with
 4: (A) collect the eggs
 EOF
+
 test_todo_session 'replace with &' << EOF
 >>> todo.sh replace 3 "thrash the hay & thresh the wheat"
 3: jump on hay
@@ -90,4 +91,44 @@ test_todo_session 'replace error' << EOF
 10: No such task.
 EOF
 
+cat /dev/null > todo.txt
+test_todo_session 'replace handling prepended date on add' <<EOF
+>>> todo.sh -t add "new task"
+1: 2009-02-13 new task
+TODO: 1 added.
+
+>>> todo.sh replace 1 this is just a new one
+1: 2009-02-13 new task
+replaced with
+1: 2009-02-13 this is just a new one
+
+>>> todo.sh replace 1 2010-07-04 this also has a new date
+1: 2009-02-13 this is just a new one
+replaced with
+1: 2010-07-04 this also has a new date
+EOF
+
+cat /dev/null > todo.txt
+test_todo_session 'replace 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 replace 1 this is just a new one
+1: (A) 2009-02-13 new task
+replaced with
+1: (A) 2009-02-13 this is just a new one
+EOF
+
+test_todo_session 'replace with prepended date replaces existing date' <<EOF
+>>> todo.sh replace 1 2010-07-04 this also has a new date
+1: (A) 2009-02-13 this is just a new one
+replaced with
+1: (A) 2010-07-04 this also has a new date
+EOF
+
 test_done