diagnostic: Fix suggested git remote commands

[bfontaine]

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git remote add origin ... doesn’t work because there’s already a remote.

[MikeMcQuaid]

Nice! Does this work both with non-existing and already existing remotes?

[bfontaine]

No, but the origin.nil? case is already taken care of in both… cases.

[apjanke]

Good catch.

[MikeMcQuaid]

Nice work @bfontaine!