ci: compare CLI and GixPython repository benchmarks

Add a separate `Backend benchmark` job using one CPython 3.12 interpreter and
two installations with released GixPython 0.2.0. Explicitly select the backend
for each manager and its workers. Prepare the fixed SHA-1/files GitPython
3.1.45 checkout through a read-only `gix.testtools` Bash fixture before timing;
its initial state includes one branch, an untracked file and an ignored directory.
Prepare Rust and cache Cargo outputs for source builds on Linux. Keep 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 completed stack passed all 14 worker smoke measurements on both backends
with matching result digests and existing CLI-process budgets. The workflow
passes actionlint and whitespace checks. Hosted execution remains unverified;
single-value local samples establish no performance conclusion.

Author: codex

.github/workflows/backend-benchmark.yml

@@ -0,0 +1,80 @@
+name: Backend benchmark
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  compare-backends:
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    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 $3}')" ge 2.52; then
+            sudo add-apt-repository --yes ppa:git-core/ppa
+            sudo apt-get update
+            sudo apt-get install --yes git
+          fi
+      - name: Prepare Rust for GixPython
+        run: |
+          rustup update stable
+          rustup default stable
+          echo "CARGO_TARGET_DIR=$RUNNER_TEMP/gix-target" >> "$GITHUB_ENV"
+      - name: Cache GixPython builds
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            ${{ runner.temp }}/gix-target
+            ~/.cache/pip
+          key: benchmark-gix-${{ runner.os }}-${{ hashFiles('gix-requirements.txt') }}
+      - name: Install separate backends with the same interpreter
+        run: |
+          python -m venv .bench-cli
+          python -m venv .bench-gix
+          .bench-cli/bin/python -m pip install . -r test/performance/benchmark-requirements.txt
+          .bench-gix/bin/python -m pip install '.[gix]' -r test/performance/benchmark-requirements.txt
+      - name: Prepare the shared read-only benchmark fixture
+        run: |
+          .bench-cli/bin/python -c 'from pathlib import Path; from gix import testtools; print("BENCH_REPO=" + str((testtools.scripted_fixture_read_only_with_args("benchmark.sh", [str(Path.cwd())]) / "repo").resolve()))' >> "$GITHUB_ENV"
+          mkdir benchmark-results
+      - name: Measure GixPython operations, opening and discovery first
+        env:
+          GIT_PYTHON_BACKEND: gix
+        run: |
+          .bench-gix/bin/python -m test.performance.bench_repository \
+            --repo "$BENCH_REPO" --expect-backend gix -o benchmark-results/gix.json
+      - name: Measure CLI operations, opening and discovery
+        env:
+          GIT_PYTHON_BACKEND: cli
+        run: |
+          .bench-cli/bin/python -m test.performance.bench_repository \
+            --repo "$BENCH_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/