[PATCH 1 of 2] merge: separate read-only mergestate into separate module

[Yuya Nishihara]

On Fri, 4 Mar 2016 12:12:04 -0800, Simon Farnsworth wrote:
> # HG changeset patch
> # User Simon Farnsworth <simonfar at fb.com>
> # Date 1456843967 0
> #      Tue Mar 01 14:52:47 2016 +0000
> # Node ID 37fe1f9d08245f7540cb6137c312ae30dbcde688
> # Parent  c7f89ad87baef87f00c507545dfd4cc824bc3131
> merge: separate read-only mergestate into separate module
> 
> This is code motion to prepare for revset predicates that read the
> mergestate; without this code motion, there would be an import loop.

Why did you separate a read-only mergestate class? It doesn't make sense to
separate read/write functions of the same storage format.

I guess the reason would be to cut the import loop from mergestate._resolve()
to filemerge, which I think worth mentioning in a commit message. Someone can
find a better way.