TL;DR: There is a bug in the command `todo-jump-to-category' from
todo-mode.el that manifests itself differently in master and in earlier
versions of Emacs due to the change to `setq-default' made in commit
f3e0bdf1b98.  A patch that fixes both manifestations is provided.

Background: As documented, `todo-jump-to-category' visits a
user-specified category of a given Todo file, and if the specified
category does not exist in the Todo file, the user is prompted whether
to add the category to the file, and if confirmed, then by default there
is a further prompt to add the first Todo item to the category.  The
command can also be invoked from outside of a todo-mode buffer, but
since it is not autoloaded, todo-mode must be loaded before it can be so
invoked.

Problem (master): Recently, I started observing aberrant behavior when
invoking `todo-jump-to-category' before visiting any Todo file in
todo-mode (I load todo-mode in my init file): upon jumping to an
existing category, I get prompted to add a Todo item, although the
category is not new.  To reproduce this:

0. HOME=/tmp/${USER} emacs -Q  (start Emacs in a clean environment)
1. Type `M-x todo-show' and at the prompt type "test RET test1 RET item1
   RET" to initialize the Todo file "test" with the category "test1"
   containing the item "item1".  The current buffer displays that
   category in todo-mode.
2. (Sanity check) Type `q' to save the newly created Todo file and bury
   the buffer, so that the current buffer is no longer a todo-mode
   buffer.  Then type `M-x todo-jump-to-category': this switches the
   buffer back to the todo-mode buffer displaying the category "test1"
   with the item "item1"; there is no prompt for a Todo item.
3. Kill the Emacs session and restart as in step 0.
4. Type `M-x load-library' and at the prompt enter `todo-mode RET'.
5. Type `M-x todo-jump-to-category' and at the prompt enter "test1" (or
   type TAB, which automatically inserts "test1" after the prompt, since
   it's the only category in the Todo file).
=> The category "test1" is displayed but now the minibuffer displays the
   prompt "Todo item: ".

Solution: The following patch fixes this, so that at step 5 the category
is displayed and there is no prompt to add an item.