Fix merge-upstream script for macOS users

[myabc]

FreeBSD-based implementations of sed require an empty argument with the -i flag.

See SO comment: https://unix.stackexchange.com/a/36805

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[myabc]

Hi @oliverguenther - would you mind checking this doesn't break anything for you?

[oliverguenther]

I'm also on Mac, so this won't help you. What we do in devops scripts is require Mac users to have gsed installed, check for its presence, and fail if you don't have it. This ensures parity, as sed has a lot of other edge cases where it doesn't behave identically, depending on the patterns.

[myabc]

@oliverguenther thanks for the context. I'll go ahead and close this PR - and install gsed instead.