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.

Remote/Git.hs

@@ -230,7 +230,7 @@ copyToRemote r key
 		-- run copy from perspective of remote
 		liftIO $ onLocal r $ do
 			ensureInitialized
-			Annex.Content.saveState `after`
+			Annex.Content.saveState True `after`
 				Annex.Content.getViaTmp key
 					(rsyncOrCopyFile params keysrc)
 	| Git.repoIsSsh r = do