Investigate avoiding cloning the repo when there are no publishable issues

[marco-c]

This should make the bot way faster when there is nothing to report, which is a great improvement for felt user experience.

CC @bhearsum

[marco-c]

Not sure if this is feasible because in before/after mode we use all issues, even pre-existing, and need to calculate hashes for them. There are a few linters that have pre-existing issues.

[marco-c]

In a subset of cases (which is actually pretty common), we can do this: #3675.