ui: move request exit handlers to global state
Since the ui objects can be created with the 'load' class method, it
is possible to lose the exit handlers information from the old ui instance. For
example, running 'test-bad-extension.t' leads to this situation where chg
creates a new ui instance which does not copy the exit handlers from the
earlier ui instance. For exit handlers, which are special cases anyways, it
probably makes sense to have a global state of the handlers. This would ensure
that the exit handlers registered once are definitely executed at the end of
the request.
Test Plan:
Ran all the tests without '--chg' option. This also fixes the
'test-bad-extension.t' with the '--chg' option.
Differential Revision: https://phab.mercurial-scm.org/D1166