- introduce the ability to reference parent / child PR relationships

- FIX https://github.com/mikepenz/release-changelog-builder-action/issues/1074
- optimize placeholder removal
- only keep placeholders in array if we have values

__tests__/transform.test.ts

@@ -266,7 +266,7 @@ pullRequestsWithLabels.push(
     repoName: 'test-repo',
     labels: ['issue', 'fix'],
     milestone: '',
-    body: 'no magic body for this matter',
+    body: 'no magic body for this matter - #1',
     assignees: [],
     requestedReviewers: [],
     approvedReviewers: [],
@@ -399,6 +399,26 @@ it('Deduplicate duplicated PRs DESC', async () => {
   )
 })
 
+it('Reference PRs', async () => {
+  const customConfig = Object.assign({}, DefaultConfiguration)
+  customConfig.categories = [
+    {
+      title: '',
+      labels: []
+    }
+  ]
+  customConfig.pr_template = "${{NUMBER}} -- ${{REFERENCED[*].number}}"
+  customConfig.reference = {
+    pattern: '.*\ \#(.).*',
+    on_property: 'body',
+    method: 'replace',
+    target: '$1'
+  }
+  expect(buildChangelogTest(customConfig, pullRequestsWithLabels)).toStrictEqual(
+    `1 -- 2\n4 -- \n3 -- \n\n`
+  )
+})
+
 it('Use empty_content for empty category', async () => {
   const customConfig = Object.assign({}, DefaultConfiguration)
   customConfig.categories = [