Restore flake8 check with ... diff

.pre-commit-config.yaml

@@ -10,6 +10,14 @@ repos:
     rev: 5.7.0
     hooks:
       - id: isort
+  - repo: https://gitlab.com/pycqa/flake8
+    rev: 3.9.0
+    hooks:
+    - id: flake8
+      # If `CI_MERGE_REQUEST_TARGET_BRANCH_SHA` env var is set then this will
+      # run flake8 on the diff of the merge request, otherwise it will run
+      # flake8 as it would usually execute via the pre-commit hook
+      entry: bash -c 'if [ -z ${CI_MERGE_REQUEST_TARGET_BRANCH_SHA} ]; then (flake8 "$@"); else (git diff $CI_MERGE_REQUEST_TARGET_BRANCH_SHA...$CI_MERGE_REQUEST_SOURCE_BRANCH_SHA | flake8 --diff); fi' --
   - repo: https://github.com/myint/rstcheck
     rev: 3f92957478422df87bd730abde66f089cc1ee19b  # commit where pre-commit support was added
     hooks: