better prevention of auto repack

Looking through the git sources (documentation is unclear),
it seems commit doesn't ever trigger git-gc, mostly fetching and merging
seems to. I cannot easily override the setting in all those places, so
instead set gc.auto in git config when initializing a repository with
the assistant.

This does mean that the user cannot set gc.auto=0 and completely avoid
repacks, as the assistant does it daily. But, it only does it after there
are 100x the default number of loose objects, so this is probably not going
to be too annoying.

Assistant/Threads/SanityChecker.hs

@@ -15,6 +15,7 @@ import Assistant.DaemonStatus
 import Assistant.Alert
 import qualified Git.LsFiles
 import qualified Git.Command
+import qualified Git.Config
 import Utility.ThreadScheduler
 import qualified Assistant.Threads.Watcher as Watcher
 import Utility.LogFile
@@ -85,8 +86,16 @@ dailyCheck = do
 	liftIO $ void cleanup
 
 	{- Allow git-gc to run once per day. More frequent gc is avoided
-	 - to avoid slowing things down. -}
-	void $ liftIO $ Git.Command.runBool [Param "gc", Param "--auto"] g
+	 - by default to avoid slowing things down. Only run repacks when 100x
+	 - the usual number of loose objects are present; we tend
+	 - to have a lot of small objects and they should not be a
+	 - significant size. -}
+	when (Git.Config.getMaybe "gc.auto" g == Just "0") $
+		liftIO $ void $ Git.Command.runBool
+			[ Param "-c", Param "gc.auto=670000"
+			, Param "gc"
+			, Param "--auto"
+			] g
 
 	return True
   where