revsets: let p1() and p2() return parents of working dir This patch makes the 'set' argument to revset functions p1() and p2() optional. If no argument is given, p1() and p2() return the first or second parent of the working directory. If the working directory is not an in-progress merge (no 2nd parent), p2() returns the empty set. For a checkout of the null changeset, both p1() and p2() return the empty set.

util: refactor opener - Don't call atomictempfile or nlinks() if the path is malformed (no basename). Let posixfile() raise IOError directly. - atomictempfile already breaks up hardlinks, no need to poke at the file with nlinks() if atomictemp. - No need to copy the file contents to break hardlinks for 'w'rite modes (w, wb, w+, w+b). Unlinking and recreating the file is faster.