Checking out a pull request reference locally

The source code checked out in a github pull request merges with its target.

When we checkout a git repository, the repository's configuration has a refspec configured so that all of the server's references in refs/heads/ maps to our local .git/refs/remotes/origin/ folder.

[remote "origin"]
        ...
	fetch = +refs/heads/*:refs/remotes/origin/*

When I configure my repository forge integration, magit adds another refspec:

[remote "origin"]
        ...
  fetch = +refs/pull/*/head:refs/pullreqs/*

Before we continue, I must explainu that GitHub maintains a dedicated branch for each pull requests where the branch of the pull request is already merged with the base.

This is what is checked out when we in the CI.

These references are stored on the server's refs/pull folder.

Now that we know there's a reference on the server that has the base branch merged, let's try to check it out.

We can't simply run git checkout refs/pull/1 since refs/pull/1 is a specific to the server, not locally.

Since it's been mapped to a local reference, we can do git checkout refs/pullreqs/1 instead!

If your pull request's branch is up to date and only has one commit, then the merged pull request branch will be linear and simple:

git log -n2 --graph --oneline
# * 34cc96194 (HEAD, origin/foobar, foobar) feat: bonjour
# * f4ce741b9 (origin/main, origin/HEAD) feat: helloworld

If you only want to see a graph of the new commits being introduced by your pull request's branch:

  git log origin/main..HEAD --graph --oneline --boundary # an 'o' indicates the merge base.

If you only want to get the files that have changed in the pull request:

  git diff $(git merge-base origin/main HEAD)..HEAD --name-only