cmdutil: comprehensively document the interface of export
I want to make some improvements here, but in order to make future
patches easier to review I want to document the current state of the
world.
tests: hide warning from test-xdg.t
The test-xdg.t test uses the system hgrc because it unsets HGRCPATH. If the
system has an extension enabled that doesn't work with the development version
of Mercurial it prints an 'extension failed to load' warning. Let's just hide
that by piping stderr to /dev/null.