3.8.0

CHANGES:

• Adapt git-mirage with awa.0.1.0 (@hannesm, #553)
• Support new version of git into tests (@dinosaure, #532, #554)
• Provide Store.read_opt for finding Git object (@craigfe, #551)
• Unify git-mirage-http and git-mirage-ssh about the authenticator (@dinosaure, @hannesm, #538, #555)