fix: Single commit validation does not factor in merge commits [#108]

.github/workflows/lint-pr-title-preview-validateSingleCommit.yml

@@ -0,0 +1,23 @@
+name: 'Lint PR title preview (current branch, validateSingleCommit enabled)'
+on:
+  pull_request:
+    types:
+      - opened
+      - edited
+      - synchronize
+
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: 12
+      - run: yarn install
+      - run: yarn build
+      - uses: ./
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          validateSingleCommit: true

src/index.js

@@ -51,16 +51,37 @@ module.exports = async function run() {
         });
 
         if (validateSingleCommit) {
-          const {data: commits} = await client.pulls.listCommits({
-            owner,
-            repo,
-            pull_number: contextPullRequest.number,
-            per_page: 2
-          });
+          const commits = [];
+          let nonMergeCommits = [];
 
-          if (commits.length === 1) {
+          for await (const response of client.paginate.iterator(
+            client.pulls.listCommits,
+            {
+              owner,
+              repo,
+              pull_number: contextPullRequest.number
+            }
+          )) {
+            commits.push(...response.data);
+
+            // GitHub does not count merge commits when deciding whether to use
+            // the PR title or a commit message for the squash commit message.
+            nonMergeCommits = commits.filter(
+              (commit) => !commit.commit.message.startsWith('Merge branch')
+            );
+
+            // We only need two non-merge commits to know that the PR
+            // title won't be used.
+            if (nonMergeCommits.length >= 2) break;
+          }
+
+          // If there is only one (non merge) commit present, GitHub will use
+          // that commit rather than the PR title for the title of a squash
+          // commit. To make sure a semantic title is used for the squash
+          // commit, we need to validate the commit title.
+          if (nonMergeCommits.length === 1) {
             try {
-              await validatePrTitle(commits[0].commit.message, {
+              await validatePrTitle(nonMergeCommits[0].commit.message, {
                 types,
                 scopes,
                 requireScope,