[boost] Trusting Subversion, reducing server load

Doing an SVN update instead of a full check-out would reduce server bandwidth considerably (and reduce test cycle time). I wonder if Ticket #4916 <https://svn.boost.org/trac/boost/ticket/4916> has something to do with the decision (not) to trust Subversion. '--force-update' doesn't wipe out intermediate files as regression.py clearly intends it, making it act like '--incremental'. I think someone should fix #4916, make '--force-update' the default, and see what that does to server load. Then, bring back source browsing if we can.