Avoid repeated location log commits when a remote is receiving files.

Done by adding a oneshot mode, in which location log changes are written to
the journal, but not committed. Taking advantage of git-annex's existing
ability to recover in this situation.

This is used by git-annex-shell and other places where changes are made to
a remote's location log.

Command/RecvKey.hs

@@ -28,7 +28,7 @@ start key = do
 	if ok
 		then do
 			-- forcibly quit after receiving one key,
-			-- and shutdown cleanly so queued git commands run
-			_ <- shutdown
+			-- and shutdown cleanly
+			_ <- shutdown True
 			liftIO exitSuccess
 		else liftIO exitFailure