Deduce primary branch name in merge script #1261

Job	Run time
build_NetCore (Debug_NetCore)	1m 3s
build_NetCore (Release_NetCore)	1m 3s
build (6.8, Debug)	1m 46s
build (6.8, Release)	2m 49s
build (6.10, Debug)	1m 57s
build (6.10, Release)	1m 51s
build (6.12, Debug)	1m 39s
build (6.12, Release)	1m 28s
build (latest, Debug)	1m 29s
build (latest, Release)	2m 13s
	17m 18s

Provide feedback