Fix detect-changed-benchmarks: handle shallow base fetch (no merge base)

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas. Opened as a draft.

Problem

.github/scripts/detect-changed-benchmarks.sh fails the Detect Changed Benchmarks job on PR re-runs with:

fatal: origin/master...HEAD: no merge base
##[error]Process completed with exit code 128.

The PR path does:

git fetch origin "${BASE_SHA}" --depth=1 2>/dev/null || true
CHANGED_FILES=$(git diff --name-only "origin/${BASE_SHA}...HEAD" -- 'benchmarks/')

The three-dot range origin/master...HEAD diffs from the merge base of the two commits, which requires a common ancestor to exist locally. But the base is re-fetched at --depth=1 (an orphan graft with no ancestors). Once master advances past the PR's branch point, the depth=1 base tip shares no ancestor with HEAD, so git aborts with "no merge base" (exit 128). That line has no || true, so under set -eo pipefail the whole step fails.

This is why it's intermittent: a freshly-dispatched PR (base == current master tip) finds a merge base, but a re-run after master moves on does not.

Observed

• Refresh DynamicalODE under v7 stack #1581 (Refresh DynamicalODE): detect passed on the 2026-05-22 dispatch, then failed on the 2026-05-23 re-run.
• Fix NBodySimulator under v7 stack: shrink liquid_argon problem size #1570 (Fix NBodySimulator): failed on the 2026-05-25 re-run.

Both fail at this exact line; it is latent for the whole v7-refresh batch on any re-run.

Fix

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 robust 2>/dev/null || true + HEAD~1 pattern already used on the push path in the same script.

CHANGED_FILES=$(git diff --name-only "origin/${BASE_SHA}...HEAD" -- 'benchmarks/' 2>/dev/null || true)
if [[ -z "${CHANGED_FILES}" ]]; then
    CHANGED_FILES=$(git diff --name-only HEAD~1 HEAD -- 'benchmarks/' 2>/dev/null || true)
fi

Verification

Reproduced the failure locally in a scratch repo recreating the CI condition (master advanced past the merge ref's base; base re-fetched at --depth=1):

• Old code: fatal: origin/master...HEAD: no merge base (exit 128) — matches CI exactly.
• New code: three-dot returns empty (suppressed), fallback returns exactly the PR's changed benchmark file.

bash -n passes on the edited script.

🤖 Generated with Claude Code