Fix pointing GitHub sources to PRs

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

Recent refactorings in git sources to use shallow clones when possible broke pointing to GitHub PRs.

Git sources are now more strict and require that the parameter the :branch to be an actual branch, and the parameter to :tag to be an actual branch. For arbitrary refs, which is what pointing to GitHub pull requests use under the hood, ref should be used.

What is your fix for the problem, implemented in this PR?

This PR changes Bundler to use ref internally for handling PR URLs given as the value for the :github parameter.

Fixes #6239.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[etiennebarrie]

It works for me. Thanks for the quick fix!

Maybe there's some kind of integration test missing showing that gem "foo", github: "https://github.com/foo/foo/pull/123" works in its entirety, from the DSL to the fetching and installing.

[deivid-rodriguez]

Definitely, otherwise we would've caught this. It did not seem super easy though, so I decided to pass on adding a spec here.

[deivid-rodriguez]

I added a realworld spec in the end, just to get the feature covered.