bzr --version should also report plugin versions (bzrlib ...)

[Ian Clatworthy]

Jari Aalto wrote:

> Plugin versions in general. John suggested that all the info,
> including bzrlib, would be moved to "bzr plugins". Somehow the version
> number of all plugins should be required. That would help e.g. out of
> date detections.

I didn't read John's email as suggesting bzrlib's version should be
returned by the plugins command, just the plugin versions. For backwards
compatibility, I'm not sure we can insist of plugins having a version
either, though we could output a warning if they don't.

Ian C.