The session is not used anymore. Instead, the browser module keeps track
of the selected base for diff, when one selects a directory entry
(subdirectory or file) and when one selects a different revision.
Also in this changeset:
* anydiff.cs: that file has been merged in browser.cs
* Browser.py: In the Revision Log, it is now possible to diff
between revisions corresponding to different paths
(which happens after move and rename operations).