doc: make sure we use our own code for generating man pages
docutils would in some situations pick up its own manpage.py instead of
doc/manpage.py. Renaming to hgmanpage.py makes it less ambiguous.
% Without -v
access log created - .hg/hgrc respected
% errors
% With -v
listening at http://localhost/ (bound to 127.0.0.1:HGPORT1)
% errors
% With -v and -p HGPORT2
listening at http://localhost/ (bound to 127.0.0.1:HGPORT2)
% errors
% With --prefix foo
listening at http://localhost/foo/ (bound to 127.0.0.1:HGPORT1)
% errors
% With --prefix /foo
listening at http://localhost/foo/ (bound to 127.0.0.1:HGPORT1)
% errors
% With --prefix foo/
listening at http://localhost/foo/ (bound to 127.0.0.1:HGPORT1)
% errors
% With --prefix /foo/
listening at http://localhost/foo/ (bound to 127.0.0.1:HGPORT1)
% errors