add excludeTargetBranch input option

README.md

@@ -72,6 +72,12 @@ You can see more info about GitHub's default token [here](https://docs.github.co
 
 default: `${{ github.token }}`
 
+### `excludeTargetBranch`
+
+When set to `true` excludes commits from the target branch. This is useful if the two branches diverged at a time before `commitlint` was used. In this case, the target branch may have old commits that are not in the source branch and which don't follow the lint rules. In this case, they'll be flagged on every check and there's nothing you can do to resolve it. In this case, turn on `excludeTargetBranch`.
+
+default: `false`
+
 ## Outputs
 
 ### `results`

action.yml

@@ -34,6 +34,17 @@ inputs:
       https://docs.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token
     default: ${{ github.token }}
     required: false
+  excludeTargetBranch:
+    description: >
+      When set to `true` excludes commits from the target branch. This is useful on merge commits of two
+      divergent branches. By default all of the commits in the target branch that are not in the source
+      branch will be linted again. Set this to `true` to avoid that.
+
+      TODO: This explanation can be made better when I better understand why `--firstParent` alone didn't resolve this.
+      To do that I need to understand what `git log <revision range>` is doing.
+    default: "false"
+    required: false
+
 outputs:
   results:
     description: The error and warning messages for each one of the analyzed commits

src/action.js

@@ -12,7 +12,7 @@ const pullRequestEvent = 'pull_request'
 const pullRequestTargetEvent = 'pull_request_target'
 const pullRequestEvents = [pullRequestEvent, pullRequestTargetEvent]
 
-const { GITHUB_EVENT_NAME, GITHUB_SHA } = process.env
+const { GITHUB_EVENT_NAME, GITHUB_SHA, GITHUB_BASE_REF } = process.env
 
 const configPath = resolve(process.env.GITHUB_WORKSPACE, getInput('configFile'))
 
@@ -78,6 +78,10 @@ function getHistoryCommits(from, to) {
     options.firstParent = true
   }
 
+  if (getInput('excludeTargetBranch') === 'true') {
+    options[`^${GITHUB_BASE_REF}`] = true
+  }
+
   if (!from) {
     options.maxCount = 1
   }