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

mercurial/commands.py

@@ -6348,7 +6348,7 b' def serve(ui, repo, **opts):'
             )
         s = wireprotoserver.sshserver(ui, repo)
         s.serve_forever()
-        sys.exit(0)
+        return
 
     service = server.createservice(ui, repo, opts)
     return server.runservice(opts, initfn=service.init, runfn=service.run)

mercurial/debugcommands.py

@@ -3376,7 +3376,6 b' def debugserve(ui, repo, **opts):'
 
     s = wireprotoserver.sshserver(ui, repo, logfh=logfh)
     s.serve_forever()
-    sys.exit(0)
 
 
 @command(b'debugsetparents', [], _(b'REV1 [REV2]'))