Prevent Dzomo from doing empty commits

[beurdouche]

As discussed with @msprotz , removing the git option --allow-empty from the build config would avoid having multiple empty commits in the history of the HaCl* repository.

[msprotz]

from what I recall git diff --exit-code didn't do the right thing

[msprotz]

and you can't just remove the option because it'll make the script fail with no good reason

[s-zanella]

This is tracked in Azure DevOps as https://msr-project-everest.visualstudio.com/Everest/_queries/edit/80/?triage=true

[tahina-pro]

Fixed in F* and miTLS (master and dev)