Add dirstate.pathto and localrepo.pathto.
Every time util.pathto is called, we have to pass the repo root and the
repo cwd.
dirstate.pathto is a simple convenience function that knows about the
root and the cwd arguments. It's still possible to pass the cwd as an
optimization.
localrepo.pathto is a convenience function that just calls
dirstate.pathto, just like localrepo.getcwd.
dirstate.pathto becomes a single point that converts most (all?) paths
from the internal representation to some OS-specific relative path for
display purposes.