Update defaultbranch to 20

Change-Id: Ib91db9223a78188667e15053bcc73931b878414e

.gitreview

 [gerrit]
-defaultbranch=master
-basebranch=master
+defaultbranch=20
 #
 # Intentional padding to ensure it is possible to point a commit
 # to an alternative gerrit server/repository without breaking