Use the HISTFILE environment variable in place of copying it to
_per_directory_history_global so that users can change the environment variable
after sourcing per-directory-history and have the global variable set correctly
This is a implementation of per directory history for zsh, some
implementations of which exist in bash[1,2]. It also implements
a per-directory-history-toggle-history function to change from using the
directory history to using the global history. In both cases the history is
always saved to both the global history and the directory history, so the
toggle state will not effect the saved histories. Being able to switch
between global and directory histories on the fly is a novel feature as far
as I am aware.
[1]: http://www.compbiome.com/2010/07/bash-per-directory-bash-history.html
[2]: http://dieter.plaetinck.be/per_directory_bash
Jim Hester