fix(ci): improve attribution of co-authors

Fix the `Co-authored-by` inclusion in commit messages so that co-authorship is properly expressed. Additionally, filter merge commits before unique authors. Previously, we would not attribute an author if their first commit in a PR was a merge commit. Finally, we remove superfluous newlines between the `Co-authored-by` lines.

Pull-Request: #4104.

Co-authored-by: Thomas Eizinger <[email protected]>

.github/mergify.yml

@@ -4,15 +4,19 @@ defaults:
       method: squash
       commit_message_template: |
         {{ title }}
-        
+
         {{ body | get_section("## Description", "") }}
-        
+
         Pull-Request: #{{ number }}.
         {# Here comes some fancy Jinja2 stuff for correctly attributing co-authorship: #}
-        {% for commit in (commits | unique(False, 'email_author')) | rejectattr("author", "==", author) %}
-          {% if commit.parents|length == 1 %}
-        Co-Authored-By: {{ commit.author }} <{{ commit.email_author }}>
-          {% endif %}
+        {%- set _ = 0 -%}
+        {%- for commit in commits -%}
+          {%- if commit.parents|length != 1 -%}
+            {%- set _ = commit.update({'merge': true}) -%}
+          {%- endif -%}
+        {%- endfor -%}
+        {%- for commit in (commits | rejectattr("merge") | unique(False, 'email_author')) | rejectattr("author", "==", author) -%}
+        Co-authored-by: {{ commit.author }} <{{ commit.email_author }}>
         {% endfor %}
         {# GitHub requires that the `Co-authored-by` lines are AT THE VERY END of a commit, hence nothing must come after this. #}