Add RepositoriesService.GetByID method.

[dmitshur]

Note: GetByID uses the undocumented GitHub API endpoint /repositories/:id.

Include license preview Accept header.

Helps #329.

This commit is based on https://github.com/sourcegraph/go-github/commit/da394ef1f53de34e9c3e5782ca27589df32b426c which has been tested in production. It has been amended by me to include Licences API header, improved documentation and commit message, with appropriate updates to the test.

I've tested it locally with the Licences API header to make sure it works, and I've confirmed that it did.

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request) and all commit authors, but as best as we can tell these commits were authored by someone else. If that's the case, please add them to this pull request and have them confirm that they're okay with these commits being contributed to Google. If we're mistaken and you did author these commits, just reply here to confirm.

[dmitshur]

@sqs, can you confirm you're okay with this commit being contributed to Google?

[sqs]

👍

[sqs]

I confirm.

[dmitshur]

@googlebot, can you check again?

[willnorris]

"need author consent" is a terminal CLA state. It will never automatically come out of this state; don't worry about it.

[dmitshur]

Thanks!

[willnorris]

Thank you guys. This will actually be pretty useful inside Google as well, so I'm glad to see this added.