Add a new user option 'ido-big-directories'.

This provides an alternative to 'ido-max-directory-size', for
directories that are statically known to be too big for Ido
completion.

* lisp/ido.el (ido-big-directories): New user option.
(ido-directory-too-big-p): Use it.

* test/lisp/ido-tests.el (ido-directory-too-big-p): New unit test.
1 job for master in 60 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #1416
test-all

01:00:01

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/progmodes/sql-tests.log
ELC lisp/progmodes/subword-tests.elc
GEN lisp/progmodes/subword-tests.log
ELC lisp/progmodes/tcl-tests.elc
GEN lisp/progmodes/tcl-tests.log
ELC lisp/progmodes/xref-tests.elc
GEN lisp/progmodes/xref-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h0m0s seconds