DEV: Fix an apparently "too modern" git command (PR #10894)

A follow-up to a follow-up. (6932a373a34a7150f8514af7edaf9048cc719a7d and 572da7a57b7ec672fcde673a5d7ac4ce555f249d) Our discours_test Docker image uses git 2.20.1 released on Dec 15, 2018. It does not support git branch --show-current. (it was added in 2.22.0)

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