ci: run commitlint to check commits (#1602)

As first step in our main "check" workflows (push, linux, mac,
windows), check all the commit messages with commitlint.

These workflows can be triggered in various ways:
pull requests, pushes, manually, or scheduled.

For (each push to) a pull request, all commits currently in the PR
branch are checked.

For a regular push, all the pushed commits are checked.

For a manual run, it seemed to check the same commits as a push (which
push ? Not sure how this works).

For a scheduled run - we'll see.

.github/workflows/linux.yml

@@ -87,6 +87,25 @@ jobs:
       with: 
         fetch-depth: 0        
 
+    #- name: Print debug output
+    #   env:
+    #     GITHUB_CONTEXT: ${{ toJson(github) }}
+    #   run: |
+    #     echo "$GITHUB_SHA"
+    #     echo "$GITHUB_REF"
+    #     echo "$GITHUB_HEAD_REF"
+    #     echo "$GITHUB_BASE_REF"
+    #     git log "$GITHUB_BASE_REF"..
+    #     bin/commitlint "$GITHUB_BASE_REF"..
+
+    - name: Check commit messages
+      env:
+        BEFORE: ${{ github.event.push.before }}
+      # for a PR this will be: origin/master..origin/$GITHUB_HEAD_REF
+      # for a push it will be: $BEFORE..
+      run: |
+        bin/commitlint ${BEFORE:-origin/master}..${GITHUB_HEAD_REF:+origin/$GITHUB_HEAD_REF}
+
     # things to be cached/restored:
 
     - name: Cache stack global package db

.github/workflows/mac.yml

@@ -57,6 +57,12 @@ jobs:
       with: 
         fetch-depth: 0        
 
+    - name: Check commit messages
+      # for a PR this will be: master..origin/$GITHUB_HEAD_REF
+      # for a push it will be: master..
+      run: |
+        bin/commitlint origin/master..${GITHUB_HEAD_REF:+origin/$GITHUB_HEAD_REF}
+
     # things to be cached/restored:
 
     - name: Cache stack global package db

.github/workflows/push.yml

@@ -50,6 +50,12 @@ jobs:
       with: 
         fetch-depth: 0        
 
+    - name: Check commit messages
+      # for a PR this will be: master..origin/$GITHUB_HEAD_REF
+      # for a push it will be: master..
+      run: |
+        bin/commitlint origin/master..${GITHUB_HEAD_REF:+origin/$GITHUB_HEAD_REF}
+
     # things to be cached/restored:
 
     - name: Cache stack global package db

.github/workflows/windows.yml

@@ -47,6 +47,12 @@ jobs:
       with: 
         fetch-depth: 0        
 
+    - name: Check commit messages
+      # for a PR this will be: master..origin/$GITHUB_HEAD_REF
+      # for a push it will be: master..
+      run: |
+        bin/commitlint origin/master..${GITHUB_HEAD_REF:+origin/$GITHUB_HEAD_REF}
+
     # things to be cached/restored:
 
     - name: Cache stack global package db