contributor: change mentions of mpm to olivia
Matt Mackall is now Olivia Mackall.
I reached out to her about changing the copyright notices to reflect this
change and she gave me the green light, so I changed everything relevant.
Differential Revision: https://phab.mercurial-scm.org/D10266
.. 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