made parentDir return a Maybe FilePath; removed most uses of it

parentDir is less safe than takeDirectory, especially when working
with relative FilePaths. It's really only useful in loops that
want to terminate at /

This commit was sponsored by Audric SCHILTKNECHT.

Assistant/Ssh.hs

@@ -233,7 +233,8 @@ genSshKeyPair = withTmpDir "git-annex-keygen" $ \dir -> do
 setupSshKeyPair :: SshKeyPair -> SshData -> IO SshData
 setupSshKeyPair sshkeypair sshdata = do
 	sshdir <- sshDir
-	createDirectoryIfMissing True $ parentDir $ sshdir </> sshprivkeyfile
+	createDirectoryIfMissing True $
+		takeDirectory $ sshdir </> sshprivkeyfile
 
 	unlessM (doesFileExist $ sshdir </> sshprivkeyfile) $
 		writeFileProtected (sshdir </> sshprivkeyfile) (sshPrivKey sshkeypair)