tests/test-mq-qsave.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

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state