ci: compare CLI and GixPython repository benchmarks

Add a separate `Backend benchmark` job using one CPython 3.12 interpreter and
two installations, with official `GixPython==0.1.0` only in the native one.
Prepare a fixed SHA-1/files checkout of GitPython 3.1.45 with one branch, an
untracked file and an ignored directory before timing. Keep local/global Git
configuration and optional locks from changing the measured workload.

Run the calibrated `pyperf` suite with GixPython first, then CLI on the same
runner. Include separate direct repository opening and nested discovery while
keeping the operation journey on a retained `Repo`. Validate result digests and
workload metadata, publish all means and
standard deviations plus `pyperf` significance reporting in the job summary,
and retain raw JSON and comparison artifacts even on failure. Timings are
observational on shared runners; execution and parity errors fail the job,
without a noisy performance threshold. New `MEASUREMENTS` entries automatically
participate in this job.

The final fixed workload passed all eleven GixPython-first and CLI-second smoke
measurements locally with matching result digests. Four comparison regression
cases passed both installations. Workflow YAML parsing, Ruff lint/format and
`git diff --check` passed. Hosted execution will follow the branch push.

Author: codex

.github/workflows/backend-benchmark.yml

@@ -0,0 +1,77 @@
+name: Backend benchmark
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  compare-backends:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+      - uses: actions/setup-python@v7
+        with:
+          python-version: "3.12"
+      - name: Ensure Git 2.52 or newer
+        run: |
+          if ! dpkg --compare-versions "$(git version | awk '{print $3}')" ge 2.52; then
+            sudo add-apt-repository --yes ppa:git-core/ppa
+            sudo apt-get update
+            sudo apt-get install --yes git
+          fi
+      - name: Install separate backends with the same interpreter
+        run: |
+          python -m venv .bench-cli
+          python -m venv .bench-gix
+          .bench-cli/bin/python -m pip install . -r test/performance/benchmark-requirements.txt
+          .bench-gix/bin/python -m pip install '.[gix]' -r test/performance/benchmark-requirements.txt
+      - name: Prepare an existing repository outside the timed operations
+        env:
+          BENCH_REPO: ${{ runner.temp }}/benchmark-repo
+          GIT_CONFIG_NOSYSTEM: "1"
+          GIT_CONFIG_GLOBAL: /dev/null
+        run: |
+          git init --object-format=sha1 --ref-format=files "$BENCH_REPO"
+          git -C "$BENCH_REPO" fetch --no-tags "$GITHUB_WORKSPACE" 6ba2c0a2f9ee7feffd7e079621c4845820180c9a
+          git -C "$BENCH_REPO" checkout -b benchmark FETCH_HEAD
+          git -C "$BENCH_REPO" config core.excludesFile /dev/null
+          git -C "$BENCH_REPO" config core.attributesFile /dev/null
+          git -C "$BENCH_REPO" config core.fsmonitor false
+          git -C "$BENCH_REPO" config core.untrackedCache false
+          git -C "$BENCH_REPO" config gc.auto 0
+          git -C "$BENCH_REPO" config maintenance.auto false
+          mkdir "$BENCH_REPO/.benchmark-cache"
+          touch "$BENCH_REPO/.benchmark-cache/probe" "$BENCH_REPO/benchmark-untracked.txt"
+          echo '.benchmark-cache/' >> "$BENCH_REPO/.git/info/exclude"
+          mkdir benchmark-results
+      - name: Measure GixPython operations, opening and discovery first
+        run: |
+          .bench-gix/bin/python -m test.performance.bench_repository \
+            --repo "$RUNNER_TEMP/benchmark-repo" --expect-backend gix -o benchmark-results/gix.json
+      - name: Measure CLI operations, opening and discovery
+        run: |
+          .bench-cli/bin/python -m test.performance.bench_repository \
+            --repo "$RUNNER_TEMP/benchmark-repo" --expect-backend cli -o benchmark-results/cli.json
+      - name: Check parity and publish comparison
+        run: |
+          .bench-gix/bin/python -m test.performance.compare_backends \
+            benchmark-results/cli.json benchmark-results/gix.json > benchmark-results/comparison.md
+          cat benchmark-results/comparison.md >> "$GITHUB_STEP_SUMMARY"
+          .bench-gix/bin/python -m pyperf compare_to \
+            benchmark-results/cli.json benchmark-results/gix.json --table --table-format md \
+            >> "$GITHUB_STEP_SUMMARY"
+      - name: Retain raw timings and comparison
+        if: always()
+        uses: actions/upload-artifact@v7
+        with:
+          name: backend-benchmark
+          path: benchmark-results/