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 black test-repo
$ cd $RUNTESTDIR/..
$ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`