`git_buf_copy_cstr` is called with user-input, and wants to sanity-check
that input.  Allow it to return a value if the input was malformed in a
way that we cannot cope.

`git_buf_sanitize` is called with user-input, and wants to sanity-check
that input.  Allow it to return a value if the input was malformed in a
way that we cannot cope.

ci: only report main branch in README status

ci: run coverity in the nightly builds

ci: more GitHub Actions

GitHub Actions is the present and the future.