mypy_primer: use target base branch for finding merge base (#14595)

Author: hauntsaninja

.github/workflows/mypy_primer.yml

@@ -48,7 +48,7 @@ jobs:
           echo "new commit"
           git rev-list --format=%s --max-count=1 $GITHUB_SHA
 
-          MERGE_BASE=$(git merge-base $GITHUB_SHA origin/master)
+          MERGE_BASE=$(git merge-base $GITHUB_SHA origin/$GITHUB_BASE_REF)
           git checkout -b base_commit $MERGE_BASE
           echo "base commit"
           git rev-list --format=%s --max-count=1 base_commit