Allows using new special remote messages when git-annex supports them,
and avoiding using them when git-annex is too old. The new INFO is one
such message.
There's also the possibility, currently unused, for the special remote's
reply to include some kind of extensions of its own.
Merging this is blocked by https://github.com/datalad/datalad/issues/2124
since it seems it will break datalad. I checked all the other special
remotes and they will be ok.
This commit was supported by the NSF-funded DataLad project.
