github/workflows: Add workflow to verify commit message format.

Using the new tools/verifygitlog.py script.

.github/workflows/commit_formatting.yml

@@ -0,0 +1,14 @@
+name: Check commit message formatting
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: '100'
+    - uses: actions/setup-python@v1
+    - name: Check commit message formatting
+      run: source tools/ci.sh && ci_commit_formatting_run

tools/ci.sh

@@ -29,6 +29,16 @@ function ci_code_formatting_run {
     tools/codeformat.py -v
 }
 
+########################################################################################
+# commit formatting
+
+function ci_commit_formatting_run {
+    git remote add upstream https://github.com/micropython/micropython.git
+    git fetch --depth=100 upstream  master
+    # For a PR, upstream/master..HEAD ends with a merge commit into master, exlude that one.
+    tools/verifygitlog.py -v upstream/master..HEAD --no-merges
+}
+
 ########################################################################################
 # code size