tests/test-contrib-emacs.t

serve: simply return instead of calling sys.exit() in `hg serve --stdio`

The shouldn't be a reason to call `sys.exit()` instead of letting the
code return normally. I've remove the call in both `hg serve` and `hg
debugserve`.

Differential Revision: https://phab.mercurial-scm.org/D9271

#require emacs
  $ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
  >  -f ert-run-tests-batch-and-exit
  Running 1 tests (*) (glob)
     passed  1/1  hg-test-mode--compilation-mode-support* (glob)
  
  Ran 1 tests, 1 results as expected* (glob)