narrow: move the code to generate a widening bundle2 to core
This is a part of moving more narrow related bits to core.
Differential Revision: https://phab.mercurial-scm.org/D4888
Mercurial's documentation is kept in reStructuredText format, which is
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http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.