Add Merge-Branch script eng/scripts

[hallipr]

Bring the merge-branch script from spring's branch into main

[hallipr]

overriding check enforcer as this isn't used by any pipelines

[hallipr]

/check-enforcer override

[check-enforcer]

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[benbp]

Nit but one possibility is to rename ErrorExit to ExitIfError and do the exit code checking within the function, ditching the if blocks.