git.github_pr_destination pr_branch parameter doesn't properly insert labels

[EliteMasterEric]

The PR branch parameter to the Github PR destination currently only supports the ${CONTEXT_REFERENCE}, and has no support for other labels.

As an example, I tried to use this configuration, to make the branch name clear.

    pr_branch = "public/copybara-${GITHUB_PR_USER}-${GITHUB_PR_NUMBER}",

However, instead of the intended result, I got public/copybara-public/copybara-__GITHUB_PR_USER_-public/copybara-__GITHUB_PR_USER_-__GITHUB_PR_NUMBER_-public/copybara-__GITHUB_PR_USER_-__GITHUB_PR_NUMBER_.

I believe the relevant code is located here:

copybara/java/com/google/copybara/git/GitHubPrDestination.java

Line 401 in 0a50f53

.resolve(e -> e.equals("CONTEXT_REFERENCE") ? contextReference : prBranch);