Fix use_github for https protocol with github PAT

[cderv]

This PR fixes #320 (use_github does not push with https protocol and GITHUB_PAT), closes #334 and contributes to #322.

As discussed, if no token is provided as argument (auth_token = NULL) in use_github, the default expected behaviour (GITHUB_PAT or GITHUB_TOKEN env var) is used through gh_token().

For use_github to work, I needed to deal with git2r cran or dev version. This is now included:

• use git2r::head for version <= 0.21.0 (currently cran version of git2r)
• use new git2r::repository_head for version > 0.21.0 (currently dev version of git2r

Lastly, I included a manual test file for use_github and https. I need to set up ssh on my computer to add ssh test. Could do it but in another PR.
I think manual test file are great for this kind of interactive 📦 like usethis. However, I am thinking that it could be improved by a Rmd test report or using testthat in manual test script. If your are insterested, I am willing to work on something.

Hope this PR suits you. Tell me what I need to adjust if necessary.

[jennybc]

I merged this manually and added a few more commits. But yours are there and I added a NEWS bullet. Thanks!

[cderv]

Perfect thanks.

I see you have simplified the manual test - I'll keep this good practice on mind for next time.

Glad I could help and contribute!

See you in the next issue or PR! 😉

[jennybc]

I see you have simplified the manual test - I'll keep this good practice on mind for next time.

Well, we don't really have standards for these. But I did decide to make it a bit more light weight, more so that anyone who visits doesn't need to read as much code. It makes things less automatic, no doubt, but I figure that is OK for a manual test.