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/Uninit.hs

@@ -57,7 +57,7 @@ cleanup = do
 	mapM_ removeAnnex =<< getKeysPresent
 	liftIO $ removeDirectoryRecursive annexdir
 	-- avoid normal shutdown
-	saveState
+	saveState False
 	inRepo $ Git.Command.run "branch"
 		[Param "-D", Param $ show Annex.Branch.name]
 	liftIO exitSuccess