Track master branch for repohooks project

Changes to repohooks will need to be applied to all branches. Instead of
cherry-picking every change made to repohooks, let's just track the
master branch instead.

BUG=b/149121748
TEST=repo init and repo sync

Change-Id: Ibe6ff4f5b7f22842838eb87a239ec379f812abb8
diff --git a/external_full.xml b/external_full.xml
index 4e0529e..73082f6 100644
--- a/external_full.xml
+++ b/external_full.xml
@@ -80,7 +80,8 @@
   <project path="src/third_party/portage-stable"
            name="third_party/overlays/portage-stable" />
   <project path="src/repohooks"
-           name="cos/repohooks" />
+           name="cos/repohooks"
+           revision="refs/heads/master" />
   <project path="src/platform/cobble"
            name="cos/cobble" />
   <project path="manifest" name="cos/manifest" >