tests/test-chg.t

chg: add fchdirx as a utility function

As part of the series to support long socket paths, we need to use fchdir and
check its result in several places. Make it a utility function.

init repo

  $ hg init foo
  $ cd foo

ill-formed config

  $ hg status
  $ echo '=brokenconfig' >> $HGRCPATH
  $ hg status
  hg: parse error at * (glob)
  [255]