help: describe new cross-branch behavior in update help text, plus cleanups

update: allow branch crossing without -c or -C, with no uncommitted changes Update will now allow crossing branches within the same named branch, when given a specific revision, if the working dir is clean, without requiring the -c or -C option. Abort if no revision is given and this would cross branches. Minor change to abort message if uncommitted changes are found. Modify test-update-branches and output to reflect the altered case. Modify test-merge5.out to reflect the altered case. Modify test-up-local-change.out with new message.