Bug: git.diff is not populated in CI on Github Actions

When trying to write logic / rules around `git.diff`, the action is hanging and this debugging / warning i added never seems to get hit.

```
onGitHub {
// Debug: Check if diff is available
val diffContent = git.diff
warn("DEBUG: Diff is ${if (diffContent.isNullOrEmpty()) "empty/null" else "available (${diffContent.length} chars)"}")
}
```

I am running in CI on Github Actions via this yml.

```
name: Danger

on:
pull_request:
types: [ synchronize, opened, reopened, edited ]

jobs:
danger-kotlin:
runs-on: ubuntu-latest
name: "Run Danger"
steps:
- name: create app token
uses: actions/create-github-app-token@v1
id: app-token
with:
app-id: ${{ vars.DANGER_APP_ID }}
private-key: ${{ secrets.DANGER_APP_PRIVATE_KEY }}
- uses: actions/checkout@v4
with:
fetch-depth: 0
- name: Danger
uses: danger/kotlin@1.3.3
with:
dangerfile: danger/Dangerfile.df.kts
args: --verbose
env:
GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
DANGER_GITHUB_API_TOKEN: ${{ steps.app-token.outputs.token }}
```

Start with danger/Dangerfile.df.kts and the supplied GitHub Actions workflow, especially actions/checkout@v4 with fetch-depth: 0 and the danger/kotlin@1.3.3 entry point. Reproduce with --verbose and inspect the onGitHub block around git.diff. Done means the action no longer hangs there and git.diff is populated or its unavailable state is handled consistently in CI.

Written by the indexing model from the issue text.