.github: only fetch the base branch when rebasing

Update the "check commits" action to only fetch the base branch that we
will be rebasing on. Otherwise every upstream branch is fetched.

.github/workflows/main.yml

@@ -105,7 +105,7 @@ jobs:
       - name: fetch and rebase on ${{ github.base_ref }}
         run: |
           git remote add upstream https://github.com/${{ github.repository }}
-          git fetch upstream
+          git fetch upstream ${{ github.base_ref }}:refs/remotes/upstream/${{ github.base_ref }}
           export GIT_COMMITTER_EMAIL="lnd-ci@example.com"
           export GIT_COMMITTER_NAME="LND CI"
           git rebase upstream/${{ github.base_ref }}