Create a way to configure git protocol and git2r credentials

[jennybc]

Gives us:

• git_protocol() / use_git_protocol() to summon or set a default protocol (so, either SSH or HTTPS)
• git2r_credentials() / use_git2r_credentials() to summon or set git2r credentials

This addresses your earlier distaste @hadley for getting and setting protocol with the same function, i.e. I've split it out and followed same design for credentials.

Clears the way for me to tackle #533 Build protocol flexibility into pr_*() functions

[jennybc]

@cderv @ijlyttle With this merged, I think we are going to have much more success with HTTPS and on Windows, because I make a git2r credential from the GitHub PAT whenever possible. Now I will use this to wire this all in to the PR flow.

But starting now, it would be great for you two to test drive this as much as possible. It's nearly impossible to test. I do some manual tests but I don't cover many scenarios.

[cderv]

First try with a ˋpr_fetch` and first error. Something does not work as intended with git2r::fetch and credentials.
I need to dig a little further to isolate the issue.

[ijlyttle]

So far, I have succeeded, no problems, on macOS with: pr_init(), pr_push(), and pr_fetch().

I'll use more of the functions today and try things with Windows.

[cderv]

ok my bad, this is working now with last pr updates. Will continue to test and open specific issues if needed.