tools/ci.sh: Fix commit msg checking when PR branch HEAD behind master.

Fixes the problem noted at
https://github.com/micropython/micropython/pull/15547#issuecomment-2434479702
which is that, because default CI HEAD for a PR is a (generated) merge
commit into the master branch's current HEAD, then if the PR branch isn't
fully rebased then the commit check runs against commits from master as
well!

Also drops running this check on push, the pull_request event is triggered
by default on open and update ("synchronized" event), which probably covers
the cases where this check should run.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

.github/workflows/commit_formatting.yml

@@ -1,6 +1,6 @@
 name: Check commit message formatting
 
-on: [push, pull_request]
+on: [pull_request]
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -12,7 +12,7 @@ jobs:
     steps:
     - uses: actions/checkout@v4
       with:
-        fetch-depth: '100'
+        fetch-depth: 100
     - uses: actions/setup-python@v5
     - name: Check commit message formatting
       run: source tools/ci.sh && ci_commit_formatting_run