fix: Single commit validation does not factor in merge commits [amann…

…n#108]

src/index.js

@@ -58,9 +58,19 @@ module.exports = async function run() {
             per_page: 2
           });
 
-          if (commits.length === 1) {
+          // 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,