docs: add macro to changelog template

templates/CHANGELOG.md.j2

@@ -4,6 +4,9 @@ All notable changes to this project will be documented in this file. This projec
 
 Available releases: <https://github.com/afuetterer/services-test4/releases>
 
+{% macro version_compare_url(prev_version, version) -%}
+    https://github.com/afuetterer/services-test4/compare/{{ prev_version }}...{{ version }}
+{%- endmacro %}
 {% if context.history.unreleased | length > 0 %}
 {# UNRELEASED #}
 ## Unreleased
@@ -16,10 +19,10 @@ Available releases: <https://github.com/afuetterer/services-test4/releases>
 {% endif %}{% endfor %}{% endfor %}
 {% endif %}
 {# RELEASED #}
-{% set base_url = 'https://github.com/afuetterer/services-test4/compare/' %}
 {% for version, release in context.history.released.items() %}
 {% if loop.nextitem %}
-{% set compare_url = base_url ~ loop.nextitem[0] + '...' ~ version %}
+{% set prev_version = loop.nextitem[0] %}
+{% set compare_url = version_compare_url(prev_version, version) %}
 ## [{{ version.as_tag() }}]({{ compare_url }}) ({{ release.tagged_date.strftime("%Y-%m-%d") }})
 {% else %}
 ## {{ version.as_tag() }} ({{ release.tagged_date.strftime("%Y-%m-%d") }})