changegroup: don't define lookupmf() until it is needed
lookupmf() is currently defined earlier than when it is needed. Future
patches further refactoring this code will be easier to read when
lookupmf() is in its new home.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html