Make PR change-detection robust to shallow base fetch (no merge base)

detect-changed-benchmarks.sh fetches the PR base at --depth=1 and then runs
a three-dot diff (origin/master...HEAD), which requires a merge base. Once
master advances past the PR's branch point, the depth=1 base graft shares no
ancestor with HEAD, so git aborts with "fatal: origin/master...HEAD: no merge
base" (exit 128) and the Detect Changed Benchmarks job fails. This broke
re-runs of e.g. #1581 and #1570.

Suppress the three-dot error and fall back to diffing the PR merge commit
against its first parent (HEAD~1 = base tip), which yields exactly the PR's
changes without needing a merge base. This mirrors the existing robust pattern
already used on the push path in the same script.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

.github/scripts/detect-changed-benchmarks.sh

@@ -20,7 +20,15 @@ source "${SCRIPT_DIR}/read-benchmark-config.sh"
 if [[ "${GITHUB_EVENT_NAME}" == "pull_request" ]]; then
     BASE_SHA="${GITHUB_BASE_REF}"
     git fetch origin "${BASE_SHA}" --depth=1 2>/dev/null || true
-    CHANGED_FILES=$(git diff --name-only "origin/${BASE_SHA}...HEAD" -- 'benchmarks/')
+    CHANGED_FILES=$(git diff --name-only "origin/${BASE_SHA}...HEAD" -- 'benchmarks/' 2>/dev/null || true)
+    if [[ -z "${CHANGED_FILES}" ]]; then
+        # The three-dot range needs a merge base, which a shallow (--depth=1)
+        # fetch of the base lacks once master advances past the PR's branch
+        # point — git then aborts with "no merge base". PR builds check out
+        # refs/pull/N/merge, so HEAD~1 is the base tip; diffing against it
+        # yields exactly the PR's changes without requiring a merge base.
+        CHANGED_FILES=$(git diff --name-only HEAD~1 HEAD -- 'benchmarks/' 2>/dev/null || true)
+    fi
 else
     # Push event: compare against the last commit that was successfully published
     # to SciMLBenchmarksOutput. This makes change detection cumulative — if a