osx: install bash and zsh completions by default
The zsh location appears to be on the default $fpath for zsh. bash, on
the other hand, appears to have no default location for completion
scripts, so we follow the lead of Apple's Git distribution and select
a semi-arbitrary place in /usr/local for the file.
.. 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