[PATCH 3 of 3 V2] memctx: allow the memlightctx thats reusing the manifest node

[Sean Farley]

Jun Wu <quark at fb.com> writes:

> Per discussion offline, I think this (a new class) approach is the cleanest
> and safest approach to reuse manifest nodes.
>
> "absorb" has an "overlaycontext" which takes an "original" context, but
> overrides some file contents. In the future we may want to have such
> overlaycontext, while the "memlightctx" is a special case where you don't
> override any file contents. Note: that should not require a resend of this
> patch to implement such overlaycontext feature right now.

Why can't we refactor memctx? I was planning on removing committablectx
in the future and just use memctx to build a commit (and even merge).