By default Unison uses the ~/.unison directory to store its cache and
profiles. However, this location is configurable through the $UNISON
environment variable. Quoting from the manual [1]:
> If the environment variable UNISON is defined, then its value will be
> used as the name of this directory.
This patch enables users to move the .unison directory and still enjoy
zsh completion for profiles.
[1] https://www.cis.upenn.edu/~bcpierce/unison/download/releases/stable/unison-manual.html#unisondir