Add host argument to github_resolve_ref()

[yutannihilation]

Fix #284

Currently, github_resolve_ref() doesn't pass host to github_GET(), so if the ref contains a branch or a PR name, it fails for GitHub Enterprise.

This PR adds host argument to github_resolve-ref() and pass host to it.

[yutannihilation]

Note that, though I believe this is the right change to make, I cannot use devtools::install_github() without dependencies = FALSE for a GitHub Enterprise that requires PATs. We can set GITHUB_PAT for only one of the GitHub Enterprise or github.com, which means the other one fails with 401 error (bad credentials)...

I almost gave up. No idea what to do to make remotes do the right thing.

[jimhester]

I don't think we can support multiple PATs without serious refactoring of the internals. I guess the assumption is if you are using github enterprise all of the github remotes should live in your enterprise instance.

[yutannihilation]

I guess the assumption is if you are using github enterprise all of the github remotes should live in your enterprise instance.

Thanks, agreed. Then, I'll try to make the same change on other places.

By the way, how about using option() instead of host argument? I feel it's a bit error-prone to pass them as arguments.

[yutannihilation]

@jimhester
Sorry, I gave this up since I found users often need to use both GitHub.com (for installing dev packages) and GitHub Enterprise (for installing internal packages) at the same time.

The problem is that install_github() checks updates for all installed packages, but some of my packages are installed from GitHub.com even when I try to install a package from GitHub Enterprise, so I get 404 errors.

So, I really need multiple PATs. On the other hand, I think this is right.

I don't think we can support multiple PATs without serious refactoring of the internals.

remotes package handles pretty complex things, so I understand that it's a reasonable decision not to support multiple PAT. I may come up with some workaround for this, but it will probably make things more complicated. So, let me close this.

(I don't intend to blame you, but just want to let you know currently it's very difficult to use install_github() for GitHub Enterprise...)