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

src/index.js

@@ -51,16 +51,41 @@ module.exports = async function run() {
         });
 
         if (validateSingleCommit) {
-          const {data: commits} = await client.pulls.listCommits({
-            owner,
-            repo,
-            pull_number: contextPullRequest.number,
-            per_page: 2
-          });
-
-          if (commits.length === 1) {
+          const commits = [];
+
+          // Pull all of the commits for the given PR from the GitHub API
+          const COMMITS_PER_PAGE = 50;
+          const MAX_PAGES = 100; // A sanity limit
+          let page = 1;
+          while (page < MAX_PAGES) {
+            const {data} = await client.pulls.listCommits({
+              owner,
+              repo,
+              pull_number: contextPullRequest.number,
+              per_page: COMMITS_PER_PAGE,
+              page
+            });
+
+            commits.push(...data);
+
+            if (data.length < COMMITS_PER_PAGE) break;
+
+            page++;
+          }
+
+          // GitHub does not count merge commits when deciding whether to use
+          // the PR title or a commit message for the squash commit message.
+          const nonMergeCommits = commits.filter(
+            (commit) => !commit.commit.message.startsWith('Merge branch')
+          );
+
+          // 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,