partially complete gcrypt remote (local send done; rest not)

This is a git-remote-gcrypt encrypted special remote. Only sending files
in to the remote works, and only for local repositories.

Most of the work so far has involved making initremote work. A particular
problem is that remote setup in this case needs to generate its own uuid,
derivied from the gcrypt-id. That required some larger changes in the code
to support.

For ssh remotes, this will probably just reuse Remote.Rsync's code, so
should be easy enough. And for downloading from a web remote, I will need
to factor out the part of Remote.Git that does that.

One particular thing that will need work is supporting hot-swapping a local
gcrypt remote. I think it needs to store the gcrypt-id in the git config of the
local remote, so that it can check it every time, and compare with the
cached annex-uuid for the remote. If there is a mismatch, it can change
both the cached annex-uuid and the gcrypt-id. That should work, and I laid
some groundwork for it by already reading the remote's config when it's
local. (Also needed for other reasons.)

This commit was sponsored by Daniel Callahan.

Types/Remote.hs

@@ -31,7 +31,7 @@ data RemoteTypeA a = RemoteType {
 	-- generates a remote of this type
 	generate :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> a (RemoteA a),
 	-- initializes or changes a remote
-	setup :: UUID -> RemoteConfig -> a RemoteConfig
+	setup :: Maybe UUID -> RemoteConfig -> a (RemoteConfig, UUID)
 }
 
 instance Eq (RemoteTypeA a) where