[RFC] revision id "pseudonyms" and converted-from revision property

[Ian Clatworthy]

Jelmer Vernooij wrote:

> Ian, does the fastexport/fastimport protocol provide enough information
> to determine a foreign revision id?

No it doesn't. The stream itself typically references commits via
abstract integers called 'marks' but even that is optional.

There are a few options though:

* a mapping from native-revision-id to mark is often saved by the
  export tool and a matching one is saved by bzr fast-import

* there's a new 'feature' feature for extending the stream with
  new metadata. We could leverage that.

Ian C.