Really fix not linting merge commits

It was silly of me to not test this properly. The merge commit should be
ignored.

Author: chmouel

.tekton/linter.yaml

@@ -65,12 +65,9 @@ spec:
               image: jorisroovers/gitlint
               workingDir: $(workspaces.source.path)
               script: |
-                set -x
                 git config --global --add safe.directory /workspace/source
-                # don't lint merge commit
-                git log -1 |grep -E -q '^Merge:' && exit 0
+                git log -1 --format=format:%s |grep -E -q '^Merge branch' && exit 0
                 gitlint --commit "$(git log --format=format:%H --no-merges -1)" --ignore "Merge branch"
-
       - name: yamllint
         generateName: "YAML Linter"
         runAfter: