Provide better error message when `--since` cannot find the reference

[mbj]

Locally --since usually works fine as developers have a full checkout of their git repository. But on CI things are different.

Most CI systems use a fetch depth of 1 these days (so only the last commit) which does not allow tun resolve expressions like HEAD~1 or branches.

Mutant can detect this cases on a CI and explicitly tell the user what to do. Which is in many cases adding more commits to the fetch depth.