Hmm, perhaps my thinking has been too revision-centric.  I'm not
really sure what other level of granularity is appropriate though.
Both notifications and commits should be generated on a "per-session"
level, so maybe I'll just ignore Arch and Mercurial (for whom revising
history is difficult, so per-session commits can be more work) for the
time being ;).

In that case, _every_ commit will be a
  notify-since <revision-id>
sort of change, so I'll just use libbe.diff :).