fix: do not allow squash! commits when merging (#32023)

While `fixup! ` is fine, `squash! ` means that the commit message needs
tweaking, which cannot be done automatically during merging (i.e. it
should be done by the PR author).

Previously, `validate-commit-message` would always allow
`squash! `-prefixed commits, which would cause problems during merging.

This commit changes `validate-commit-message` to make it configurable
whether such commits are allowed and configures the
`gulp validate-commit-message` task, which is run as part of the `lint`
job on CI, to not allow them.

NOTE: This new check is disabled in the pre-commit git hook that is used
      to validate commit messages, because these commits might still be
      useful during development.

PR Close #32023

tools/validate-commit-message/validate-commit-message.spec.js

@@ -144,5 +144,38 @@ describe('validate-commit-message.js', () => {
         expect(errors).toEqual([]);
       });
     });
+
+    describe('(squash)', () => {
+
+      it('should strip the `squash! ` prefix and validate the rest', () => {
+        const errorMessageFor = header =>
+            `INVALID COMMIT MSG: ${header}\n => ERROR: The commit message header does not match the format of ` +
+            '\'<type>(<scope>): <subject>\' or \'Revert: "<type>(<scope>): <subject>"\'';
+
+        // Valid messages.
+        expect(validateMessage('squash! feat(core): add feature')).toBe(VALID);
+        expect(validateMessage('squash! fix: a bug', false)).toBe(VALID);
+
+        // Invalid messages.
+        expect(validateMessage('squash! fix a typo', false)).toBe(INVALID);
+        expect(validateMessage('squash! squash! fix: a bug')).toBe(INVALID);
+        expect(errors).toEqual([
+          errorMessageFor('squash! fix a typo'),
+          errorMessageFor('squash! squash! fix: a bug'),
+        ]);
+      });
+
+      describe('with `disallowSquash`', () => {
+
+        it('should fail', () => {
+          expect(validateMessage('fix: something', true)).toBe(VALID);
+          expect(validateMessage('squash! fix: something', true)).toBe(INVALID);
+          expect(errors).toEqual([
+            'INVALID COMMIT MSG: squash! fix: something\n' +
+                ' => ERROR: The commit must be manually squashed into the target commit',
+          ]);
+        });
+      });
+    });
   });
 });