prevent exporttree=yes on remotes that don't support exports

Don't allow "exporttree=yes" to be set when the special remote
does not support exports. That would be confusing since the user would
set up a special remote for exports, but `git annex export` to it would
later fail.

This commit was supported by the NSF-funded DataLad project.

Remote/GCrypt.hs

@@ -52,14 +52,15 @@ import Utility.Gpg
 import Utility.SshHost
 
 remote :: RemoteType
-remote = RemoteType {
-	typename = "gcrypt",
+remote = RemoteType
+	{ typename = "gcrypt"
 	-- Remote.Git takes care of enumerating gcrypt remotes too,
 	-- and will call our gen on them.
-	enumerate = const (return []),
-	generate = gen,
-	setup = gCryptSetup
-}
+	, enumerate = const (return [])
+	, generate = gen
+	, setup = gCryptSetup
+	, exportSupported = exportUnsupported
+	}
 
 chainGen :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> Annex (Maybe Remote)
 chainGen gcryptr u c gc = do