ci: check out PR head commits in workflows (#21835)

## Why

PR CI should test the exact commit that was pushed to the PR branch. By
default, GitHub's `pull_request` event checks out a synthetic merge
commit from `refs/pull/<number>/merge`, so the tested tree can include
an implicit merge with the current base branch instead of matching the
pushed head SHA.

Using the PR head SHA makes each check result correspond to a concrete
commit the author submitted. This also behaves better for stacked PR
workflows, including Sapling stacks and other Git stack tooling: a
middle PR's head commit already contains the lower stack changes in its
tree, without pulling in commits above it or GitHub's temporary merge
ref.

## What Changed

- Set every `actions/checkout` in `pull_request` workflows under
`.github/workflows` to use `github.event.pull_request.head.sha` on PR
events and `github.sha` otherwise.
- Updated `blob-size-policy` to compare
`github.event.pull_request.base.sha` and
`github.event.pull_request.head.sha`, since it no longer checks out
GitHub's merge commit where `HEAD^1`/`HEAD^2` represented the PR range.

## Verification

- Parsed the edited workflow YAML files with Ruby.
- Checked that every checkout block in the `pull_request` workflows has
the PR-head `ref`.

.github/workflows/bazel.yml

@@ -58,6 +58,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
       - name: Check rusty_v8 MODULE.bazel checksums
@@ -152,6 +153,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
       - name: Prepare Bazel CI
@@ -237,6 +239,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
       - name: Prepare Bazel CI
@@ -326,6 +329,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
       - name: Prepare Bazel CI

.github/workflows/blob-size-policy.yml

@@ -10,6 +10,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           fetch-depth: 0
           persist-credentials: false
 
@@ -18,8 +19,8 @@ jobs:
         shell: bash
         run: |
           set -euo pipefail
-          echo "base=$(git rev-parse HEAD^1)" >> "$GITHUB_OUTPUT"
-          echo "head=$(git rev-parse HEAD^2)" >> "$GITHUB_OUTPUT"
+          echo "base=${{ github.event.pull_request.base.sha }}" >> "$GITHUB_OUTPUT"
+          echo "head=${{ github.event.pull_request.head.sha }}" >> "$GITHUB_OUTPUT"
 
       - name: Check changed blob sizes
         env:

.github/workflows/cargo-deny.yml

@@ -16,6 +16,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
       - name: Install Rust toolchain

.github/workflows/ci.yml

@@ -14,6 +14,7 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
       - name: Verify codex-rs Cargo manifests inherit workspace settings

.github/workflows/codespell.yml

@@ -20,6 +20,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
       - name: Annotate locations with typos
         uses: codespell-project/codespell-problem-matcher@b80729f885d32f78a716c2f107b4db1025001c42 # v1.1.0

.github/workflows/rust-ci.yml

@@ -16,6 +16,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           fetch-depth: 0
           persist-credentials: false
       - name: Detect changed paths (no external action)
@@ -64,6 +65,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
       - uses: dtolnay/rust-toolchain@a0b273b48ed29de4470960879e8381ff45632f26 # 1.93.0
         with:
@@ -82,6 +84,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
       - uses: dtolnay/rust-toolchain@a0b273b48ed29de4470960879e8381ff45632f26 # 1.93.0
       - uses: taiki-e/install-action@44c6d64aa62cd779e873306675c7a58e86d6d532 # v2.62.49
@@ -101,6 +104,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
       - uses: dtolnay/rust-toolchain@a0b273b48ed29de4470960879e8381ff45632f26 # 1.93.0
       - name: Install nightly argument-comment-lint toolchain
@@ -179,6 +183,7 @@ jobs:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         if: ${{ steps.argument_comment_lint_gate.outputs.run == 'true' }}
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
       - name: Run argument comment lint on codex-rs via Bazel
         if: ${{ steps.argument_comment_lint_gate.outputs.run == 'true' }}

.github/workflows/sdk.yml

@@ -15,6 +15,7 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
       - name: Install Linux bwrap build dependencies

.github/workflows/v8-canary.yml

@@ -42,6 +42,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
       - name: Set up Python
@@ -78,6 +79,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
       - name: Set up Bazel