master: add exclusiveGroupPermissions = push

Only allow the "Master pushers" group to push to master.

Related: SYS#6432
diff --git a/project.config b/project.config
index 482f452..442e00b 100644
--- a/project.config
+++ b/project.config
@@ -25,6 +25,7 @@
 	exclusiveGroupPermissions = create forgeAuthor push
 [access "refs/heads/master"]
 	push = group Master pushers
+	exclusiveGroupPermissions = push
 [access "refs/heads/*"]
 	create = group Administrators
 	create = group Project Owners