immediate queue flushing when annex.queuesize=1

Previously, it only flushed when the queue got larger than 1.

Also, make the queue auto-flush when items are added, rather than needing
to be flushed as a separate step. This simplifies the code and make it more
efficient too, as it avoids needing to read the queue out of the state to
check if it should be flushed.

Git/Queue.hs

@@ -137,7 +137,7 @@ merge origq newq = origq
 
 {- Is a queue large enough that it should be flushed? -}
 full :: Queue -> Bool
-full (Queue cur lim  _) = cur > lim
+full (Queue cur lim  _) = cur >= lim
 
 {- Runs a queue on a git repository. -}
 flush :: Queue -> Repo -> IO Queue