lint: Fix COMMIT_RANGE issues

ci/lint/06_script.sh

@@ -8,21 +8,24 @@ export LC_ALL=C
 
 set -ex
 
-if [ -n "$LOCAL_BRANCH" ]; then
-  # To faithfully recreate CI linting locally, specify all commits on the current
-  # branch.
-  COMMIT_RANGE="$(git merge-base HEAD master)..HEAD"
-elif [ -n "$CIRRUS_PR" ]; then
+if [ -n "$CIRRUS_PR" ]; then
   COMMIT_RANGE="HEAD~..HEAD"
-  echo
-  git log --no-merges --oneline "$COMMIT_RANGE"
-  echo
-  test/lint/commit-script-check.sh "$COMMIT_RANGE"
+  if [ "$(git rev-list -1 HEAD)" != "$(git rev-list -1 --merges HEAD)" ]; then
+    echo "Error: The top commit must be a merge commit, usually the remote 'pull/${PR_NUMBER}/merge' branch."
+    false
+  fi
 else
-  COMMIT_RANGE="SKIP_EMPTY_NOT_A_PR"
+  # Otherwise, assume that a merge commit exists. This merge commit is assumed
+  # to be the base, after which linting will be done. If the merge commit is
+  # HEAD, the range will be empty.
+  COMMIT_RANGE="$( git rev-list --max-count=1 --merges HEAD )..HEAD"
 fi
 export COMMIT_RANGE
 
+echo
+git log --no-merges --oneline "$COMMIT_RANGE"
+echo
+test/lint/commit-script-check.sh "$COMMIT_RANGE"
 RUST_BACKTRACE=1 "${LINT_RUNNER_PATH}/test_runner"
 
 if [ "$CIRRUS_REPO_FULL_NAME" = "bitcoin/bitcoin" ] && [ "$CIRRUS_PR" = "" ] ; then

ci/lint/container-entrypoint.sh

@@ -14,7 +14,7 @@ export PATH="/python_build/bin:${PATH}"
 export LINT_RUNNER_PATH="/lint_test_runner"
 
 if [ -z "$1" ]; then
-  LOCAL_BRANCH=1 bash -ic "./ci/lint/06_script.sh"
+  bash -ic "./ci/lint/06_script.sh"
 else
   exec "$@"
 fi