Use GitHub's PR refs (pull/NUMBER/head) instead of fetching branch
by name. This works for both same-repo and fork PRs because GitHub
automatically creates these refs in the base repository for all PRs.
The branch name doesn't exist on origin for fork PRs, causing:
fatal: couldn't find remote ref <branch-name>
Using pull/${entityNumber}/head:${branchName} fetches the PR head
and creates a local branch with the correct name.
Fixes issues with tag mode failing on fork PRs.
