move pre-commit hook back to travis

.travis.yml

@@ -25,6 +25,38 @@ env:
 
 jobs:
   include:
+    - name: pre-commit
+      if: type != pull_request
+      language: python
+      python: 3.7
+      # There are too many files in the repo that have formatting issues. We'll
+      # disable these checks for now when pushing directly (but still run these
+      # on Pull Requests!).
+      env: SKIP=end-of-file-fixer,trailing-whitespace,clang-format,eslint
+      cache:
+        directories:
+          - $HOME/.cache/pre-commit
+      before_install:
+        - pip install pre-commit==2.5.1
+      script:
+        - pre-commit run --all-files
+      addons: []
+
+    - name: pre-commit-pr
+      if: type == pull_request
+      language: python
+      python: 3.7
+      cache:
+        directories:
+          - $HOME/.cache/pre-commit
+      before_install:
+        - pip install pre-commit==2.5.1
+      script:
+        # HEAD is the not yet integrated PR merge commit +refs/pull/xxxx/merge
+        # HEAD^1 is the PR target branch and HEAD^2 is the HEAD of the source branch
+        - pre-commit run --from-ref HEAD^1 --to-ref HEAD --show-diff-on-failure
+      addons: []
+
     - name: Ubuntu/gcc/SCons build
       compiler: gcc
       # Ubuntu Bionic build prerequisites