fix: Consider merge commits for single commit validation (#127)

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

Closes #108

With a recent change, GitHub no longer factors in merge commits added by the "update branch" button into whether to use the PR title. If a dev submits a PR with a single commit, then presses the "update branch" button, action-semantic-pull-request will allow the branch to be merged, however GitHub will still use the commit title rather than the PR title.

* chore: Lint fix

* chore: Add comments to explain logic

* chore: Fix typo

* Minor rephrasing of comments

* Fix lint

Co-authored-by: Jan Amann <jan@amann.me>

src/index.js

@@ -58,7 +58,17 @@ 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.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, {
                 types,