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

[Jun Wu]

Excerpts from Sean Farley's message of 2016-11-17 13:41:54 -0800:
> 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).

Then you will either:

  a) worry about the "consistency" of "files" and "manifestnode" - people
     (ex. new programmer) passing inconsistent "files" and "manifestnode"
     will be able to add buggy content to the repo.

or:

  b) have perf issues - if you calculate "files" from "manifestnode" and
     its parent - it's slower than just getting "files" from the changelog.

Therefore a new class taking a "changelog node" makes perfect sense. It
can generate "manifestnode" and "files" efficiently and have no perf or
consistency issues.