Fix a bug in the GN roll script.

tools/gn/bin/roll_gn.py

Index: tools/gn/bin/roll_gn.py
diff --git a/tools/gn/bin/roll_gn.py b/tools/gn/bin/roll_gn.py
index 1e9dec9e79d594fa0c6410dad55fe602e5cf5680..ae9489fde0cab87451f0c482ffee160c9787e29b 100644
--- a/tools/gn/bin/roll_gn.py
+++ b/tools/gn/bin/roll_gn.py
@@ -376,8 +376,11 @@ class GNRoller(object):
     return 0
 
   def MovetoLastHead(self):
-    _, out, _ = self.Call('git reflog -1')
-    m = re.match('moving from ([^\s]+)', out)
+    # When this is called, there will be a commit + a checkout as
+    # the two most recent entries in the reflog, assuming nothing as
+    # modified the repo while this script has been running.
+    _, out, _ = self.Call('git reflog -2')
+    m = re.search('moving from ([^\s]+)', out)
     last_head = m.group(1)
     self.Call('git checkout %s' % last_head)