fix(ci): set merge attribute to false for non-merge commits

The previous version of this commit-message template had a bug where `merge` was unset for non-merge commits and thus the template rendering failed. See https://github.com/libp2p/rust-libp2p/pull/4128/checks?check_run_id=14597190466 for example.

Pull-Request: #4130.

Author: thomaseizinger

.github/mergify.yml

@@ -13,6 +13,8 @@ defaults:
         {%- for commit in commits -%}
           {%- if commit.parents|length != 1 -%}
             {%- set _ = commit.update({'merge': true}) -%}
+          {%- else -%}
+            {%- set _ = commit.update({'merge': false}) -%}
           {%- endif -%}
         {%- endfor -%}
         {%- for commit in (commits | rejectattr("merge") | unique(False, 'email_author')) | rejectattr("author", "==", author) -%}