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.
% convert while testing all possible outputs
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
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'221c3fdaf24df5f14c0a64c597581e2eacfb47bb branch\xc3\xa9e\n'
'7a40952c2db29cf00d9e31df3749e98d8a4bdcbf branch\xc3\xa9\n'