bug#31559: 26.1; Debugger and ERT backtraces are not abbreviated

[Noam Postavsky]

Gemini Lasswell <gazally@runbox.com> writes:

> In Emacs 25, the backtraces produced by debug.el and ert.el
> abbreviated long lists and deeply nested forms with "...".  This
> doesn't happen in Emacs 26.

Oh, I've had the impression that backtrace printing was a bit slow
sometimes, and this is probably why.

> Here is a patch that makes cl-prin1 behave like prin1 with respect to
> print-level and print-length, except for hash tables (because prin1
> prints hash table items and cl-prin1 doesn't, although maybe it
> should).

Looks good to me, unfortunately it's probably a bit too late for 26.1
though.