Check for fixup commits on CI (#3742)

I keep merging PRs that still have fixup commits on them! This will make
it impossible to do so

- **PR Description**

- **Please check if the PR fulfills these requirements**

* [ ] Cheatsheets are up-to-date (run `go generate ./...`)
* [ ] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [ ] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [ ] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [ ] Docs have been updated if necessary
* [x] You've read through your own file changes for silly mistakes etc

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see https://github.com/jesseduffield/lazygit/releases/tag/v0.40.0 for
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.github/workflows/ci.yml

@@ -219,3 +219,25 @@ jobs:
           CODACY_PROJECT_TOKEN=${{ secrets.CODACY_PROJECT_TOKEN }} \
             bash <(curl -Ls https://coverage.codacy.com/get.sh) report \
             --force-coverage-parser go -r coverage.out
+
+  check-for-fixups:
+    runs-on: ubuntu-latest
+    if: github.ref != 'refs/heads/master'
+    steps:
+      # See https://github.com/actions/checkout/issues/552#issuecomment-1167086216
+      - name: "PR commits + 1"
+        run: echo "PR_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+
+      - name: "Checkout PR branch and all PR commits"
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.ref }}
+          fetch-depth: ${{ env.PR_FETCH_DEPTH }}
+
+      - name: "Fetch the other branch with enough history for a common merge-base commit"
+        run: |
+          git fetch origin ${{ github.event.pull_request.base.ref }}
+
+      - name: Check for fixups
+        run: |
+          ./scripts/check_for_fixups.sh ${{ github.event.pull_request.base.ref }}

scripts/check_for_fixups.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+base_ref=$1
+
+# Determine the base commit
+base_commit=$(git merge-base HEAD origin/"$base_ref")
+
+# Check if base_commit is set correctly
+if [ -z "$base_commit" ]; then
+    echo "Failed to determine base commit."
+    exit 1
+fi
+echo "Base commit: $base_commit"
+
+# Get commits with "fixup!" in the message from base_commit to HEAD
+commits=$(git log -i -P --grep "fixup\!" --format="%h %s" "$base_commit..HEAD")
+
+if [ -z "$commits" ]; then
+    echo "No fixup commits found."
+    exit 0
+else
+    echo "Fixup commits found:"
+    echo "$commits"
+    exit 1
+fi