fix: access diff directly to enable support to private repositories

[lfsevergnini]

What's new

When comparing commits, add the "Accept: application/vnd.github.v3.diff" header so that Github's API returns the raw diff instead.

That allows Oktokit to get the raw diff even for private repositories, which is not possible accessing the regular diff_url.

Further info: https://stackoverflow.com/questions/40393117/getting-file-diff-with-github-api

This solves the problem found in #12.

[villesau]

Very good catch! I noticed the same issue with our private repos but didn't manage fix that yet. Thanks for this @lfsevergnini !