fix gpg pipeline stall

Bug introduced in d1da9cf221, where
I removed a forkProcess when feeding gpg. Feeding it from a thread
solves the bug.

Utility/Gpg.hs

@@ -64,7 +64,7 @@ passphraseHandle :: [CommandParam] -> String -> IO L.ByteString -> (Handle -> IO
 passphraseHandle params passphrase a b = do
 	-- pipe the passphrase into gpg on a fd
 	(frompipe, topipe) <- createPipe
-	_ <- forkIO $ do
+	void $ forkIO $ do
 		toh <- fdToHandle topipe
 		hPutStrLn toh passphrase
 		hClose toh
@@ -76,6 +76,7 @@ passphraseHandle params passphrase a b = do
 		withBothHandles createProcessSuccess (proc "gpg" params') go
 	where
 		go (to, from) = do
+			void $ forkIO $ do
 				L.hPut to =<< a
 				hClose to
 			b from

doc/bugs/transferkey_fails_due_to_gpg.mdwn

@@ -43,3 +43,9 @@ The key which is used for the remote is password protected. The GnuPG Agent asks
 When adding a new rsync-remote with encryption set to 'none' (therefore disabled), everything works, so it really seems a gpg issue.
 
 How can I help debugging?
+
+> Thanks, I reproduced a deadlock in the gpg code, which was introduced
+> with some of my earlier changes to use threading. No released version of
+> git-annex was affected, and I have developed a fix, which works for me
+> and is now committed to master. Marking this [[done]]; please do check
+> that my fix works for you! --[[Joey]]