Unbreak the history file.
Add a revert command which runs svn revert and removes any leftovers.
Clean up and simplify the svn checkout / update code.
Factor out common code for the revert and update commands.
revert before updating
Don't run svn upgrade / cleanup just for svn version.
Set MAKESYSPATH before building to ensure we use the correct set of .mk
files right from the start.

Document MAKESYSPATH and a number of other envars we set.
Bump Dd

Noticed by:	bz@