feat!: use github event payload and API to list commits resolves #456

That way we guarantee we're linting the same commits that appear on github.

BREAKING CHANGE: "firstParent" option has been removed

action.yml

@@ -3,24 +3,18 @@ description: Lints Pull Request commit messages with commitlint
 author: Wagner Santos
 inputs:
   configFile:
-    description: Commitlint config file. If the file doesn't exist, config-conventional settings will be
+    description:
+      Commitlint config file. If the file doesn't exist, config-conventional settings will be
       loaded as a fallback.
     default: ./commitlint.config.js
     required: false
-  firstParent:
-    description: >
-      When set to true, we follow only the first parent commit when seeing a merge commit.
-      More info in git-log docs
-      https://git-scm.com/docs/git-log#Documentation/git-log.txt---first-parent
-    default: "true"
-    required: false
   failOnWarnings:
     description: Whether you want to fail on warnings or not
-    default: "false"
+    default: 'false'
     required: false
   failOnErrors:
     description: Whether you want to fail on errors or not
-    default: "true"
+    default: 'true'
     required: true
   helpURL:
     description: Link to a page explaining your commit message convention
@@ -28,7 +22,7 @@ inputs:
     required: false
   commitDepth:
     description: When set to a valid Integer value - X, considers only the latest X number of commits.
-    default: ""
+    default: ''
     required: false
   token:
     description: >