ci,fix: Resolve crash when running samples in PR from fork (#189)

* fix: Forward --changed-samples-only-from argument to diff function

* fix: Use SHA instead of a ref

* fix: Check out base ref in addition to target

* ci: Specify that only changed samples are run

Author: kdestin

.github/workflows/run-samples.yml

@@ -59,6 +59,7 @@ jobs:
     # Require manual approval if initiator is not a public member of Azure-Samples
     environment: ${{ needs.check-if-external.outputs.environment }}
     steps:
+      - uses: actions/checkout@v4
       - uses: actions/checkout@v4
         with:
           ref: ${{ github.event.pull_request.head.sha || github.ref }}
@@ -81,6 +82,6 @@ jobs:
               --parameters principalType=ServicePrincipal \
               --parameters principalId="$principalId" \
               -o json > deployment.json
-      - name: Run samples
+      - name: Run Changed Samples
         run:
-            pytest --changed-samples-only-from ${{ github.base_ref }}
+            pytest --changed-samples-only-from ${{ github.event.pull_request.base.sha }}

.infra/pytest_plugins/changed_samples/src/pytest_changed_samples/plugin.py

@@ -84,8 +84,8 @@ def get_diff_paths_function(config: pytest.Config) -> Callable[[], Iterable[Path
     """
     if config.getoption(opt_var(WORKING_TREE_CHANGES_OPTION)):
         return get_all_modified_paths
-    if config.getoption(opt_var(PR_CHANGES_OPTION)):
-        return get_branch_diff_paths
+    if ref := config.getoption(opt_var(PR_CHANGES_OPTION)):
+        return lambda: get_branch_diff_paths(ref)
 
     return lambda: ()