[MERGE] Fix #296620 by allowing plugins instead of requiring them.

[Martin Pool]

Martin Pool has voted approve.
Status is now: Approved
Comment:
Seems reasonable to me, but others may wish to comment.

I'm surprised bzr passes on pqm without this, since bzrtools should not 
be present...

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3Cm2hc6ekdff.fsf%40free.fr%3E
Project: Bazaar