Add the <Email>...</Email> element in Update.xml for each commit
reported. This field was defined by Dart but never really used.
Distributed version control systems use author name and email
instead of a user id, so now it makes sense to use this field.
This creates cmCTestBZR to drive CTest Update handling on bzr-based work
trees. Currently we always update to the head of the remote tracking
branch (bzr pull), so the nightly start time is ignored for Nightly
builds. A later change will address this. Patch from Tom Vercauteren.
See issue #6857.