Update git submodules

* Update plugins/hooks from branch 'stable-2.16'
  to 57c8e6261016e1cc233ea41f19fa5bdf187df8a7
  - Fix and clarify documentation of ref-update and commit-received hooks
    
    - Clarify behavior of ref-update when multiple commits are pushed, i.e.
      that it is only called once for the entire ref update.
    
    - Fix the documentation of commit-received which states that it is called
      for changes pushed for review. In fact it is also called for direct
      pushes, but unlike ref-update it is called once for each commit.
    
    Change-Id: Ie3e2dcab37eeb5d0ed06e05322e72d3fc73925fd
    
  - Correct documentation of ref-update hook regarding output
    
    The output of the hook is only returned when the update is rejected,
    not regardless of the return code as stated in the doc.
    
    Change-Id: I93483f44e31e7055c302c0758084de868d1c4f93
    
diff --git a/plugins/hooks b/plugins/hooks
index e912780..57c8e62 160000
--- a/plugins/hooks
+++ b/plugins/hooks
@@ -1 +1 @@
-Subproject commit e9127800c4efe8ad9601c7b1152783cd12a55bc2
+Subproject commit 57c8e6261016e1cc233ea41f19fa5bdf187df8a7