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/Git.hs

@@ -128,7 +128,7 @@ repoAvail r
 	| Git.GCrypt.isEncrypted r = do
 		g <- gitRepo
 		liftIO $ do
-			er <- Git.GCrypt.encryptedRepo g r
+			er <- Git.GCrypt.encryptedRemote g r
 			if Git.repoIsLocal er || Git.repoIsLocalUnknown er
 				then catchBoolIO $
 					void (Git.Config.read er) >> return True