Retrieve commit message in a GH Action triggered on pull request

I want to create a GitHub Actions workflow triggered on pull_request. The workflow should get the last commit message in the Pull Request and check it against a given regular expression.

The part I am struggling on is actually getting the last commit message.

My workflow looks like this:

---
name: CI-PR-Check

on:
  pull_request:

jobs:
  build:
    runs-on: ubuntu-22.04
    steps:
      - name: Checkout
        uses: actions/checkout@v3
 
      - name: Check commit message
        shell: bash
        run: |
          echo "commit message validation"
          message=$(git log --pretty=%B -n 1)
          python ${{ github.action_path }}/commitcheck.py "$message"

I expected the line message=$(git log --pretty=%B -n 1) to get me the message of the last commit, but I get the message from the pull request, so something like Merge <some sha> into <other sha>.

Using GitHub webhook events, I am able to get the sha of the commit that I am interested in using sha="${{ github.event.pull_request.head.sha }}". However, it is still not possible to get the commit message with git log --format=%B -n 1 $sha. I get an error fatal: bad object <my sha>.

Is there any way to get my commit message ?

As mentioned in a comment, actions/checkout is shallow by default, so the first step is to set fetch-depth to 0:

- name: Checkout
  uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # v3.6.0
  with:
    fetch-depth: 0

I settled on this to retrieve all the commits in the PR:

# retrieve pr id
IFS=$'\/' read -r prId _ <<< $GITHUB_REF_NAME

# fetch all commits
prCommits=`gh pr view $prId --json commits | jq '.commits | length'`
fetchDepthToPrBase=`expr $prCommits + 2`
git fetch --no-tags --prune --progress --no-recurse-submodules --deepen=$fetchDepthToPrBase

# retrive the commits of the PR
commits="$(gh pr view $prId --json commits)"