MEDIUM: use go-git for base ref resolution, update docs

Author: aiharos

.github/workflows/main.yml

@@ -19,19 +19,5 @@ jobs:
         with:
           fetch-depth: 0
 
-      - uses: actions/checkout@v2
-        with:
-          ref: master
-
-      - name: Set up Go
-        uses: actions/setup-go@v2
-        with:
-          go-version: 1.15
-
-      # Runs a set of commands using the runners shell
-      - name: Run a multi-line script
-        run: |
-          export
-          git show-ref
-          git clone -b git_envs https://github.com/aiharos/github-actions.git
-          cd github-actions/check-commit; go run check.go "../../."
+      - name: check-commit
+        uses: docker://aiharos/check-commit:latest

check-commit/README.md

@@ -50,14 +50,14 @@ steps:
     uses: actions/checkout@v2
     with:
       fetch-depth: 0
-  - name: Create base branch reference
-    uses: actions/checkout@v2
-    with:
-      ref: main
   - name: check-commit
     uses: docker://haproxytech/check-commit:TAG
 ```
-Here we instruct `checkout@v2` action to fetch the repo history, as well as the main branch of the repo. This creates git refs in the checked-out repository so that the check-commit can do a merge-base operation against the main and the feature branch. Modify the main repo name from this example to reflect what is the main repo name in your own repository.
+Here we instruct `checkout@v2` action to fetch the repo history.
+
+Check-commit can work on `pull_request` events by inspecting all commit messages between a merge-base commit of the the target branch (usually main, looked up from `$GITHUB_BASE_REF`) and the tip of the feature branch (`$GITHUB_REF`) branch. If the operation is a `push` only the last commit is checked.
+
+For this to work, the `origin/$GITHUB_BASE_REF` ref has to be a part of the history checked out by `actions/checkout@v2`, so `fetch-depth: 0` is can be used as a safe option to fetch the complete repo history. Setting this to a fixed number will limit how many commits are allowed in a Pull Request to that many commits and will speed up the checkout for large repos.
 
 ## Example configuration
 

check-commit/check.go

@@ -261,7 +261,7 @@ func hashesFromRefs(repo *git.Repository, repoEnv *gitEnv) ([]*plumbing.Hash, []
 	refStrings = append(refStrings, repoEnv.Ref)
 
 	if !(repoEnv.EnvName == GITHUB && repoEnv.Event == "push") { // for Github push we only have the last commit
-		refStrings = append(refStrings, repoEnv.Base)
+		refStrings = append(refStrings, fmt.Sprintf("refs/remotes/origin/%s", repoEnv.Base))
 	}
 
 	hashes := make([]*plumbing.Hash, 0, 2)