Add composite action to forbid merge commits in Pull Requests.

Author: motlin

.github/workflows/pull-request.yml

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+on: pull_request
+
+jobs:
+  check-merge-commits:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Run Forbid Merge Commits Action
+        uses: ./

README.md

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+# "Forbid merge commits" GitHub Action
+
+This action enforces clean git history that looks like this:
+
+<img width="141" src="https://github.com/motlin/forbid-merge-commits-action/assets/244258/6b1bbe24-e6bd-43c4-8f1a-3d8e3691435f">
+
+It fails on Pull Requests that include merge commits.
+
+This rule is designed to prevent developers from merging the default branch _into_ their branch as a way of making it up-to-date. This creates [foxtrot commits](https://blog.developer.atlassian.com/stop-foxtrots-now/) and confusing git log graphs.
+
+This rule is also designed to prevent using GitHub's "Update Branch" button, which merges the base branch _into_ the source branch. Instead, use the pull-down "Rebase Branch" button which rebases the source branch _onto_ the base branch. This is a common problem if "Always suggest updating pull request branches" is enabled, because "Update Branch" is the default choice in the pull-down and [it cannot be disabled or changed](https://github.com/orgs/community/discussions/12032).
+
+## Handling failure messages
+
+If this action fails due to the presence of merge commits, it will print out the offending commits and fail the workflow. To resolve this, you will need to rebase your branch onto the base branch.
+
+If the "Rebase Branch" button is available, you can use that.
+
+<img width="335" src="https://github.com/motlin/forbid-merge-commits-action/assets/244258/dbb60788-3908-41f9-81db-4ba568d97fd5">
+
+Otherwise, you can rebase your branch locally using the following commands:
+
+```bash
+git pull <upstream-remote> <base-branch> --rebase
+git push --force-with-lease origin <branch_name>
+```
+
+The `<upstream-remote>` is usually named `upstream` or `origin`. The `<base-branch>` is usually named `main` or `master`.
+
+## When this Action is not applicable
+
+Sometimes including merge commits in Pull Requests is acceptable. For example, when maintaining a library it is common to fix problems on stable branches and merge the fixes into the default branch. In such cases, you may want to configure this action to run only on some branches, or remove it entirely.
+
+## How to add this action to your repository
+
+This action is designed trigger only `on: pull_request` events. To add this action to your repository, you can add it to an existing workflow file that triggers only on Pull Requests, or create a new workflow file. 
+
+To add this action to your repository, you need to create a new workflow file in the `.github/workflows/` directory of your repository.
+
+Here is an example `.github/workflows/pull-request.yml` workflow.
+
+```yaml
+on: pull_request
+
+jobs:
+  forbid-merge-commits:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Run Forbid Merge Commits Action
+        uses: motlin/forbid-merge-commits-action@main
+```
+
+## Recommended settings
+
+This action is designed to be used with the settings "Allow merge commits" and "Always suggest updating pull request branches."
+
+<img width="808" src="https://github.com/motlin/forbid-merge-commits-action/assets/244258/138ab7c1-6c14-4b1b-8bc4-a61fc3f6cfb6">
+
+## How is this different from the "Require Linear History" status check?
+
+"Require linear history" does not allow merges at all. It is used with "Squash and Merge" or "Rebase and Merge" and creates a completely linear history.
+
+This workflow is meant to be used with the "Merge" button on Pull Requests. These merge commits let us see who clicked the merge button, and which commits were grouped together into a single Pull Request with multiple commits.
+
+## Prior art
+
+This action is similar to [cyberark/enforce-rebase](https://github.com/cyberark/enforce-rebase), which runs using a deprecated version of Node. This action is implemented as a Composite Action using yaml, which is easier to keep up-to-date.

action.yml

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+name: 'Forbid Merge Commits'
+description: 'A composite action to forbid merge commits in pull requests.'
+branding:
+  icon: 'git-merge'
+  color: 'purple'
+
+runs:
+  using: "composite"
+  steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+
+    - name: Check for merge commits
+      run: |
+        set -Eeuo pipefail
+
+        MERGE_COMMITS=$(git log --merges --format=%H origin/${{github.base_ref}}..refs/remotes/pull/${{github.event.pull_request.number}}/merge^2 --)
+        if [ -n "$MERGE_COMMITS" ]; then
+
+          echo '::error title=Found merge commits::Found merge commits. Refer to https://github.com/motlin/forbid-merge-commits-action#handling-failure-messages for guidance.'
+
+          echo -e '## Found merge commits.\n\nRefer to [handling failure messages](https://github.com/motlin/forbid-merge-commits-action#handling-failure-messages) for guidance.\n' >> $GITHUB_STEP_SUMMARY
+
+          for COMMIT_HASH in $MERGE_COMMITS; do
+            echo "::warning::Merge commit: $COMMIT_HASH"
+            echo "::group::git show $COMMIT_HASH"
+            git show $COMMIT_HASH
+            echo '::endgroup::'
+
+            echo ''                                       >> $GITHUB_STEP_SUMMARY
+            echo '```console'                             >> $GITHUB_STEP_SUMMARY
+            git show $COMMIT_HASH                         >> $GITHUB_STEP_SUMMARY
+            echo '```'                                    >> $GITHUB_STEP_SUMMARY
+          done
+
+          exit 1
+        else
+          echo 'Success: No merge commits found'
+        fi
+      shell: bash