help: fix double word usage "most" was used twice. (I fixed a grammar error before timeless spotted it!)

setup: move cffi stuff to mercurial/cffi This patch moves all setup*cffi stuff to mercurial/cffi to make the root directory cleaner. The idea was from mpm [1]: > It seems like we could have a fair amount of cffi definitions, and > cluttering the root directory (or mercurial/) with them is probably not > a great long-term solution. We could probably add a cffi/ directory > under mercurial/ to parallel pure/. [1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-July/086442.html

manifest: remove manifest.add and add memmfctx.write This removes one more dependency on the manifest class by moving the write functionality onto the memmanifestctx classes and changing the one consumer to use the new API. By moving the write path to a manifestctx, we now give the individual manifests control over how they're read and serialized. This will be useful in developing new manifest formats and storage systems.

context: add manifestctx property on changectx This allows us to access the manifestctx for a given commit. This will be used in a later patch to be able to copy the manifestctx when we want to make a new commit.

manifest: add copy to mfctx classes This adds copy functionality to the manifestctx classes. This will be used in an upcoming diff to copy a manifestctx during commit so we can modify the manifest before committing.

manifest: introduce memmanifestctx and memtreemanifestctx This introduces two new classes to represent in-memory manifest instances. Similar to memchangectx, this lets us prepare a manifest in memory, then in a future patch we will add the apis that can commit this in memory structure.

manifestctx: add _revlog() function The `self._repo.manifestlog._revlog` code is getting copy and pasted a lot in manifestctx. Let's make it a function so it can be reused. This will make future patches cleaner too.

manifest: remove manifest.find As part of removing dependencies on manifest, this drops the find function and fixes up the two existing callers to use the equivalent apis on manifestctx.

changegroup: remove remaining uses of repo.manifest The remaining uses of repo.manifest in the changegroup module are treating the manifest exclusively as a revlog, so let's replace them with instances of the revlog directly. This is part of dropping all dependencies on repo.manifest in favor of repo.manifestlog.

treemanifest: fix a "treeinmem" case f2c5b9d48b29 (manifest: make treemanifestctx store the repo, 2016-10-18) broke most tests when run with treeinmem=True. The treeinmem mode can not be enabled by the user, so this did not break anything in practice, but it's useful to have it working for testing the treemanifest code.