Make Code-Review+2 label sticky

This saves time by allowing people to submit after addressing
minor comments from the review that generated a +2 instead of
having to wait for another approval.  The external Go repo had
the same setting enabled already.

Suggested-by: Rob Pike <r@google.com>

project.config

diff --git a/project.config b/project.config
index 47edc22..c0a4296 100644
--- a/project.config
+++ b/project.config
@@ -49,6 +49,7 @@
 	function = MaxWithBlock
 	defaultValue = 0
 	copyMinScore = true
+	copyMaxScore = true
 	copyAllScoresOnTrivialRebase = true
 	value = -2 This shall not be merged
 	value = -1 I would prefer this is not merged as is