lint-and-test: Check commit style for branch commits.

.github/workflows/lint-and-test.yml

@@ -134,6 +134,31 @@ jobs:
       - name: Run lint-docstring
         run: ./tools/lint-docstring
 
+  gitlint:
+    runs-on: ubuntu-latest
+    name: Lint - Commit text formatting
+    steps:
+      - name: 'PR commits +1'
+        run: echo "PR_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+      - name: 'Checkout PR branch and all PR commits'
+        uses: actions/checkout@v3
+        with:
+          persist-credentials: false
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: ${{ env.PR_FETCH_DEPTH }}
+      - uses: actions/setup-python@v4
+        with:
+          python-version: 3.7
+          cache: 'pip'
+          cache-dependency-path: 'setup.py'
+      - name: FIXME Install more minimally!
+        run: pip install .[dev]
+      - name: Run gitlint
+        run:
+          git fetch /~https://github.com/zulip/zulip-terminal main;
+          #git show FETCH_HEAD; git show ${{ github.event.pull_request.head.sha }};
+          gitlint --commits FETCH_HEAD..${{ github.event.pull_request.head.sha }} --debug
+
   base_pytest:
     runs-on: ubuntu-latest
     name: Install & test - CPython 3.7 (ubuntu), codecov