Specify refspec.

Author: KLMatlock

.github/workflows/on-pr.yml

@@ -47,7 +47,7 @@ jobs:
           # Fetch the pull request' base branch so towncrier will be able to
           # compare the current branch with the base branch.
           # Source: https://github.com/actions/checkout/#fetch-all-branches.
-          git fetch --no-tags origin +refs/heads/${BASE_BRANCH}:refs/remotes/origin/${BASE_BRANCH}
-          towncrier check --compare-with origin/${BASE_BRANCH}
+          git fetch --no-tags origin +refs/heads/main:refs/remotes/origin/main
+          towncrier check --compare-with origin/main
         env:
           BASE_BRANCH: ${{ github.base_ref }}