I have a need to be able to record the transaction number of the last change on
my build stream when I build so that if we later need to we can always rebuild
from that point, similar to the revision number already provided by the SVN tools.

It looks from the logs like this information is already queried, but is not made
available to the build scripts; alternately (and additionally) if the stream and
depot information were made available the build script could query for that
information on its own.

Ideally, all of this information would be available as environment variables to
the script.