Variable: undo-outer-limit-function

Function to call when an undo list exceeds `undo-outer-limit'.
This function is called with one argument, the current undo list size
for the most recent command (since the last undo boundary).
If the function returns t, that means truncation has been fully handled.
If it returns nil, the other forms of truncation are done.

Garbage collection is inhibited around the call to this function,
so it must make sure not to do a lot of consing.