Prepare tools/r8_release.py for branch 8.13

Change-Id: I7b97b2ffc34404cdafd119579e51046e10f3caa4

tools/r8_release.py

diff --git a/tools/r8_release.py b/tools/r8_release.py
index f6b916b..598ff46 100755
--- a/tools/r8_release.py
+++ b/tools/r8_release.py
@@ -17,7 +17,7 @@
 
 import utils
 
-R8_DEV_BRANCH = '8.12'
+R8_DEV_BRANCH = '8.13'
 R8_VERSION_FILE = os.path.join('src', 'main', 'java', 'com', 'android', 'tools',
                                'r8', 'Version.java')
 THIS_FILE_RELATIVE = os.path.join('tools', 'r8_release.py')