update: better logic and messages for updates - complain about attempts to merge with ancestor - when updating, differentiate between - crossing named branches with no local changes (jump) - crossing named branches with local changes (complain) - nonlinear update on the same named branch, no changes (complain some more) - nonlinear update on the same named branch, changes (different complaining)

hgweb: fix breakage in python < 2.5 introduced in 2c370f08c486