[PATCH] merge: add resolve --prep, which outputs conflicted comparison files

[Jun Wu]

Excerpts from Durham Goode's message of 2017-02-24 15:42:34 -0800:
> Maybe I don't understand your proposal.  The current merge-tools are 
> invoked once per file.  Would this python merge-tool be invoked in some 
> other way where it's given the full conflict state at the beginning then 
> it can do what it wants?  So we'd special case python merge tools to be 

Actually it's a good question. I think it's more consistent if being called
per-file. But I see that'll be problematic about "when is it the last file?"
That probably requires an extra parameter about how many files are left.

> a little different?  We could just define a internal merge tool called 
> 'internal:json' or something and skip the whole extensibility thing.

That's possible. Although I'm not sure if it's a good idea or not. Since we
have ":dump" already, maybe name it something like ":dump-json". I guess the
most difficulty part to reach agreement is to define the json schema - the
merge state includes things like executable bit, symlink, etc. So I'd rather
to put this outside core from the beginning.

> Also, how does the merge tool communicate to the caller the json 
> results? Write it to a file? Or does the merge tool have the ability to 
> print to stdout?

It has ui and repo.wvfs so it could do whatever it wants.

Therefore the signature could be something like:

  def mergefunc(ui, repo, basefctx, localfctx, otherfctx, index, total):
      raise ... # merge failure
      return None # alternative, merge failure
      return fctx-like # merge success

If we want to do the ":dump-json" thing, it could be like:
  
  state = {}
  def merge(....):
      if index == 0:
          state.clear()
      state[basefctx.path()] = ....
      if index == total:
          ui.write(json.dumps(state))
      return None # report failure