better probing for gcrypt repositories using new --check option

Now can tell if a repo uses gcrypt or not, and whether it's decryptable
with the current gpg keys.

This closes the hole that undecryptable gcrypt repos could have before been
combined into the repo in encrypted mode.

Remote/GCrypt.hs

@@ -50,7 +50,7 @@ gen :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> Annex (Maybe Remot
 gen gcryptr u c gc = do
 	g <- gitRepo
 	-- get underlying git repo with real path, not gcrypt path
-	r <- liftIO $ Git.GCrypt.encryptedRepo g gcryptr
+	r <- liftIO $ Git.GCrypt.encryptedRemote g gcryptr
 	let r' = r { Git.remoteName = Git.remoteName gcryptr }
 	(mgcryptid, r'') <- liftIO $ getGCryptId r'
 	-- doublecheck that local cache matches underlying repo's gcrypt-id