[MERGE] Add add_commit_revprops hook so plugins can set their own revprops.

[Robert Collins]

Robert Collins has voted tweak.
Status is now: Semi-approved
Comment:
Looked fine except the comments and code don't match up.

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C1235076782.10502.134.camel%40flash%3E
Project: Bazaar