add -q to git symbolic-ref call

Avoids a warning message from git when HEAD doesn't exist. Which it won't
when eg, git-annex is used in a submodule just cloned with
git clone --recursive. In this case, a specific ref is checked out and
there's no HEAD yet.

The code already returned Nothing in this case, so no behavior change other
than not showing the warning. And git-annex operates fine in this
situation.

Git/Branch.hs

@@ -37,7 +37,7 @@ current r = do
 {- The current branch, which may not really exist yet. -}
 currentUnsafe :: Repo -> IO (Maybe Git.Ref)
 currentUnsafe r = parse . firstLine
-	<$> pipeReadStrict [Param "symbolic-ref", Param $ fromRef Git.Ref.headRef] r
+	<$> pipeReadStrict [Param "symbolic-ref", Param "-q", Param $ fromRef Git.Ref.headRef] r
   where
 	parse l
 		| null l = Nothing