serve: add chdir command for --daemon-postexec

For chgserver, it probably needs a chdir to /. This patch adds chdir command
support for --daemon-postexec so chg client can make use of it.

mercurial/cmdutil.py

@@ -835,6 +835,8 b' def service(opts, parentfn=None, initfn='
             if inst.startswith('unlink:'):
                 lockpath = inst[7:]
                 os.unlink(lockpath)
+            elif inst.startswith('chdir:'):
+                os.chdir(inst[6:])
             elif inst != 'none':
                 raise error.Abort(_('invalid value for --daemon-postexec: %s')
                                   % inst)