Since there is a special callback for assigning the name from the
search box, and the callback doesn't call an operator or something
else that would do an undo push, I think the solution is to do it
manually here.
Another option would be adding the button flag "UI_BUT_UNDO", which
isn't set by default for search buttons, but that gives us ugly names for
the undo steps, so an explicit push is better in this case.
Differential Revision: https://developer.blender.org/D11190
991a1cdf5d