bookmarks: more robust parsing of bookmarks file

merge with stable

revsets: catch type error on tip^p1(tip) (issue2884)

revsets: do the right thing with x^:y (issue2884) Given an operator ^ that's either postfix or infix and an operator : that's either prefix or infix, the parser can't figure out the right thing to do. So we rewrite the expression to be sensible in the optimizer.