Use the correct base commit for change determination (#16857)

`base.sha` appears to be the commit of the base branch when the pull
request was opened, not the base commit that's used to construct the
test merge commit — which can lead to incorrect "determine changes"
results where commits made to the base ref since the pull request are
opened are included in the results.

We use `git merge-base` to find the correct sha, as I don't think that
GitHub provides this. They provide `merge_commit_sha` but my
understanding is that is equivalent to the actual merge commit we're
testing in CI.

I tested this locally on an example pull request. I don't think it's
worth trying to reproduce a specific situation here.

---------

Co-authored-by: Alex Waygood <alex.waygood@gmail.com>

.github/workflows/ci.yaml

@@ -45,10 +45,20 @@ jobs:
           fetch-depth: 0
           persist-credentials: false
 
+      - name: Determine merge base
+        id: merge_base
+        env:
+          BASE_REF: ${{ github.event.pull_request.base.ref || 'main' }}
+        run: |
+          sha=$(git merge-base HEAD "origin/${BASE_REF}")
+          echo "sha=${sha}" >> "$GITHUB_OUTPUT"
+
       - name: Check if the parser code changed
         id: check_parser
+        env:
+          MERGE_BASE: ${{ steps.merge_base.outputs.sha }}
         run: |
-          if git diff --quiet ${{ github.event.pull_request.base.sha || 'origin/main' }}...HEAD -- ':Cargo.toml' ':Cargo.lock' ':crates/ruff_python_trivia/**' ':crates/ruff_source_file/**' ':crates/ruff_text_size/**' ':crates/ruff_python_ast/**' ':crates/ruff_python_parser/**' ':python/py-fuzzer/**' ':.github/workflows/ci.yaml'; then
+          if git diff --quiet "${MERGE_BASE}...HEAD" -- ':Cargo.toml' ':Cargo.lock' ':crates/ruff_python_trivia/**' ':crates/ruff_source_file/**' ':crates/ruff_text_size/**' ':crates/ruff_python_ast/**' ':crates/ruff_python_parser/**' ':python/py-fuzzer/**' ':.github/workflows/ci.yaml'; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else
               echo "changed=true" >> "$GITHUB_OUTPUT"
@@ -56,8 +66,10 @@ jobs:
 
       - name: Check if the linter code changed
         id: check_linter
+        env:
+          MERGE_BASE: ${{ steps.merge_base.outputs.sha }}
         run: |
-          if git diff --quiet ${{ github.event.pull_request.base.sha || 'origin/main' }}...HEAD -- ':Cargo.toml' ':Cargo.lock' ':crates/**' ':!crates/red_knot*/**' ':!crates/ruff_python_formatter/**' ':!crates/ruff_formatter/**' ':!crates/ruff_dev/**' ':!crates/ruff_db/**' ':scripts/*' ':python/**' ':.github/workflows/ci.yaml'; then
+          if git diff --quiet "${MERGE_BASE}...HEAD" -- ':Cargo.toml' ':Cargo.lock' ':crates/**' ':!crates/red_knot*/**' ':!crates/ruff_python_formatter/**' ':!crates/ruff_formatter/**' ':!crates/ruff_dev/**' ':!crates/ruff_db/**' ':scripts/*' ':python/**' ':.github/workflows/ci.yaml'; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else
               echo "changed=true" >> "$GITHUB_OUTPUT"
@@ -65,8 +77,10 @@ jobs:
 
       - name: Check if the formatter code changed
         id: check_formatter
+        env:
+          MERGE_BASE: ${{ steps.merge_base.outputs.sha }}
         run: |
-          if git diff --quiet ${{ github.event.pull_request.base.sha || 'origin/main' }}...HEAD -- ':Cargo.toml' ':Cargo.lock' ':crates/ruff_python_formatter/**' ':crates/ruff_formatter/**' ':crates/ruff_python_trivia/**' ':crates/ruff_python_ast/**' ':crates/ruff_source_file/**' ':crates/ruff_python_index/**' ':crates/ruff_python_index/**' ':crates/ruff_text_size/**' ':crates/ruff_python_parser/**' ':scripts/*' ':python/**' ':.github/workflows/ci.yaml'; then
+          if git diff --quiet "${MERGE_BASE}...HEAD" -- ':Cargo.toml' ':Cargo.lock' ':crates/ruff_python_formatter/**' ':crates/ruff_formatter/**' ':crates/ruff_python_trivia/**' ':crates/ruff_python_ast/**' ':crates/ruff_source_file/**' ':crates/ruff_python_index/**' ':crates/ruff_python_index/**' ':crates/ruff_text_size/**' ':crates/ruff_python_parser/**' ':scripts/*' ':python/**' ':.github/workflows/ci.yaml'; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else
               echo "changed=true" >> "$GITHUB_OUTPUT"
@@ -74,8 +88,10 @@ jobs:
 
       - name: Check if the fuzzer code changed
         id: check_fuzzer
+        env:
+          MERGE_BASE: ${{ steps.merge_base.outputs.sha }}
         run: |
-          if git diff --quiet ${{ github.event.pull_request.base.sha || 'origin/main' }}...HEAD -- ':Cargo.toml' ':Cargo.lock' ':fuzz/fuzz_targets/**' ':.github/workflows/ci.yaml'; then
+          if git diff --quiet "${MERGE_BASE}...HEAD" -- ':Cargo.toml' ':Cargo.lock' ':fuzz/fuzz_targets/**' ':.github/workflows/ci.yaml'; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else
               echo "changed=true" >> "$GITHUB_OUTPUT"
@@ -83,8 +99,10 @@ jobs:
 
       - name: Check if there was any code related change
         id: check_code
+        env:
+          MERGE_BASE: ${{ steps.merge_base.outputs.sha }}
         run: |
-          if git diff --quiet ${{ github.event.pull_request.base.sha || 'origin/main' }}...HEAD -- ':**/*' ':!**/*.md' ':crates/red_knot_python_semantic/resources/mdtest/**/*.md' ':!docs/**' ':!assets/**' ':.github/workflows/ci.yaml'; then
+          if git diff --quiet "${MERGE_BASE}...HEAD" -- ':**/*' ':!**/*.md' ':crates/red_knot_python_semantic/resources/mdtest/**/*.md' ':!docs/**' ':!assets/**' ':.github/workflows/ci.yaml'; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else
               echo "changed=true" >> "$GITHUB_OUTPUT"
@@ -92,8 +110,10 @@ jobs:
 
       - name: Check if there was any playground related change
         id: check_playground
+        env:
+          MERGE_BASE: ${{ steps.merge_base.outputs.sha }}
         run: |
-          if git diff --quiet ${{ github.event.pull_request.base.sha || 'origin/main' }}...HEAD -- ':playground/**'; then
+          if git diff --quiet "${MERGE_BASE}...HEAD" -- ':playground/**'; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else
               echo "changed=true" >> "$GITHUB_OUTPUT"