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.
adding b rev offset length base linkrev nodeid p1 p2 0 0 3 0 0 1e88685f5dde 000000000000 000000000000