hg serve: call setconfig on the parentui
It makes sense to do this on the "global" ui object, since command line
options should affect the whole process.
This should fix hg serve --style=gitweb inside a repo (part of issue253).
abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'