mergestate: add a function to return the number of unresolved files Note that unlike the other functions, this is based on the persistent mergestate.

mergestate: add a method to return updated/merged/removed counts This will not only allow us to remove a bunch of duplicate code in applyupdates in an upcoming patch, it will also allow the resolve interface to be a lot simpler: it doesn't need to return the dirstate action to applyupdates.

mergestate._resolve: don't return the action any more This is a partial backout of an earlier diff -- now that we're storing the results in a dict, we don't actually need this any more.

mergestate._resolve: store return code and action for each file We're going to need this to compute (a) updated/merged/unresolved counts, and (b) actions to perform on the dirstate.