Incrmntl. Tests: remove call to merge-base, replace with branch diff (#2866)
Incrmntl. Tests: remove call to merge-base, replace with branch diff (#2866)

* remove merge-base call, replace with just diff of branches

* use git diff <defaultbranch> without self-branch param

ali-ince/akka.net,amichel/akka.net,Silv3rcircl3/akka.net,Silv3rcircl3/akka.net,ali-ince/akka.net,heynickc/akka.net,amichel/akka.net,simonlaroche/akka.net,simonlaroche/akka.net,heynickc/akka.net