changegroup3: move treemanifest support into _unpackmanifests() By putting the treemanifest code in _unpackmanifests(), _addchangegroupfiles() will only be about files again, and we get a nice symmetry between _packmanifests() and _unpackmanifest(). The immediate benefit to me is that remotefilelog should not need to be updated to work with treemanifests. It should also make server.validate and progress output easier to get right. Probably bundlerepo too.

changegroup3: add empty chunk separating directories and files Remotefilelog overrides changegroup._addchangegroupfiles(), assuming it is about files, which seems like a natural assumption. However, in changegroup3, directory manifests are sent in the files section of the changegroup. These naturally make remotefilelog unhappy. The fact that the directories are not separated from the files (although they do come before the files) also makes server.validate harder to implement. Since we read one chunk at a time from the steam, once we have found a file (non-directory) entry in the stream, we would have to push the read data back into the stream, or otherwise refactor the code. It will be easier if we add an empty chunk after all directory manifests. This change adds that empty chunk, although we don't yet take advantage of it on the reading side. We will soon move the tree manifest stuff out of _addchangegroupfiles() and into _unpackmanifests().

changegroup3: introduce experimental.changegroup3 boolean config In order to give us the freedom to change the changegroup3 format, let's hide it behind an experimental config. Since it is required by treemanifests, that will override the cg3 config.

changegroup: hide packermap behind methods This is to prepare for hiding changegroup3 behind a config option.

merge with stable

context: check for differing flags a little earlier This makes it clearer that a unchanged file whose flags have changed will be reported as a modification. Also test this.