dirstate: add dedicated function for updating data of a file `dirstate.normal()` is too generic to be a user facing function for that. This is a part of effort to refactor dirstate APIs and make them clearer. Differential Revision: https://phab.mercurial-scm.org/D11076

dirstate: add a `update_file` function This function is the other side of the `set_tracked`/`set_untracked` API revamp. It is to be used when the dirstate is changing its parents during and update or a merge. It states all the information we know about the file so that the dirstate can update its internal data. Unlike the `set_tracked`/`set_untracked` it has not regards for the information previously tracked in the tristate. Differential Revision: https://phab.mercurial-scm.org/D11075

resolve: use the `parentchange` context manager to apply merge action In an ideal world, we would not have to do that. However, we are miles away from being ready to not have to do it. So we add this context manager alongside a long comment. This will help use to get to the point were have two distinct API with strict rules about when to call them. Differential Revision: https://phab.mercurial-scm.org/D11074