Overriding site-wide plugins with user-installed plugins

[Jonathan Lange]

Hello,

Recently I had cause to override the site install of the PQM plugin
with a home directory install of the latest development code.

It turns out that Bazaar won't let me do that.
bzrlib.plugin.set_plugins_path() explicitly ensures that
site-installed plugins trump user-installed plugins. To work around,
I've had to abuse my root privileges and move the plugin from
site-packages.