Merge #3884 Deduce primary branch name in merge script #1263

Job	Run time
build (6.8, Debug)	1m 59s
build (6.8, Release)	2m 43s
build (6.10, Debug)	1m 41s
build (6.10, Release)	1m 25s
build (6.12, Debug)	1m 38s
build (6.12, Release)	2m 0s
build (latest, Debug)	1m 52s
build (latest, Release)	1m 59s
build_NetCore (Debug_NetCore)	52s
build_NetCore (Release_NetCore)	51s
	17m 0s

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