Avoid “Failure while executing; git config --replace-all homebrew.devcmdrun true exited with 255.” error.

[reitermarkus]

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This often happens when running multiple dev commands in parallel since it tries to lock the .git/config file, so check if the current value is already correct.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Makes sense, thanks @reitermarkus!