Fix gate step to include deletions, improve merge-base error handling, and clarify quoting comment

This fixes three issues in the simulation tests workflow: the gate step's git diff now includes deletions so PRs that only delete files are not incorrectly skipped, the merge-base computation is properly wrapped to catch and annotate failures, and the alienv quoting comment now clearly explains the hazard.

- Gate step now uses git diff without --diff-filter=AMR to include deletions
- merge-base failure is assigned to a variable first, checked, then used
- Comment updated to clearly state that a space or "$" in the workspace path would break or double-evaluate

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: sawenzel

.github/workflows/sim-tests.yml

@@ -46,7 +46,15 @@ jobs:
             echo "::error title=Cannot resolve diff head::pull request head commit $HEAD_SHA does not resolve in this checkout"
             exit 1
           }
-          echo "sha=$(git merge-base "$BASE_SHA" "$HEAD_SHA")" >> "$GITHUB_OUTPUT"
+          merge_base=$(git merge-base "$BASE_SHA" "$HEAD_SHA") || {
+            echo "::error title=Cannot compute diff base::git merge-base failed"
+            exit 1
+          }
+          [ -n "$merge_base" ] || {
+            echo "::error title=Cannot compute diff base::merge base is empty"
+            exit 1
+          }
+          echo "sha=$merge_base" >> "$GITHUB_OUTPUT"
 
       - name: Skip when not relevant or opted into the source build
         id: gate
@@ -56,7 +64,7 @@ jobs:
           HEAD_SHA: ${{ github.event.pull_request.head.sha }}
         run: |
           set -eu
-          changed=$(git diff --diff-filter=AMR --name-only "$BASE_SHA" "$HEAD_SHA")
+          changed=$(git diff --name-only "$BASE_SHA" "$HEAD_SHA")
           if grep -qx 'test/needs-o2-dev' <<< "$changed" ; then
             echo "::notice title=Skipped::this pull request touches test/needs-o2-dev, so it is tested by build/O2DPG/sim/o2dev against O2 dev instead"
             echo "skip=true" >> "$GITHUB_OUTPUT"
@@ -115,10 +123,9 @@ jobs:
         run: |
           set -eu
           # Everything after "-c" is joined into one string and re-evaluated
-          # by the CVMFS alienv via "bash -c \"$*\"", so this quoting is only
-          # applied once here and then discarded. Safe for the runner's
-          # actual workspace path, but a space or "$" in that path away from
-          # breaking.
+          # by the CVMFS alienv via "bash -c \"$*\"". Quoting here is applied
+          # once then discarded, so it is safe for the runner's workspace path,
+          # but a path with a space or "$" would break or double-evaluate.
           /cvmfs/alice.cern.ch/bin/alienv setenv "O2PDPSuite/$O2PDPSUITE_TAG" -c \
             env O2DPG_ROOT="$PWD" O2DPG_MC_CONFIG_ROOT="$PWD" \
                 O2DPG_TEST_REPO_DIR="$PWD" \