Fix merge-base calculation for change determination (#573)

See astral-sh/ruff#16857

Author: zanieb

.github/workflows/linux.yml

@@ -171,13 +171,15 @@ jobs:
 
       - name: Check if the `pythonbuild` crate changed
         id: check-pythonbuild
+        env:
+          BASE_REF: ${{ github.event.pull_request.base.ref || 'main' }}
         run: |
-          if git diff --quiet ${{ github.event.pull_request.base.sha || 'origin/main' }}...HEAD -- ':src/*.rs'; then
+          merge_base=$(git merge-base HEAD "origin/${BASE_REF}")
+          if git diff --quiet "${merge_base}...HEAD" -- ':src/*.rs'; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else
               echo "changed=true" >> "$GITHUB_OUTPUT"
           fi
-          
 
   build-0:
     needs:

.github/workflows/macos.yml

@@ -81,12 +81,16 @@ jobs:
 
       - name: Check if the `pythonbuild` crate changed
         id: check-pythonbuild
+        env:
+          BASE_REF: ${{ github.event.pull_request.base.ref || 'main' }}
         run: |
-          if git diff --quiet ${{ github.event.pull_request.base.sha || 'origin/main' }}...HEAD -- ':src/*.rs'; then
+          merge_base=$(git merge-base HEAD "origin/${BASE_REF}")
+          if git diff --quiet "${merge_base}...HEAD" -- ':src/*.rs'; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else
               echo "changed=true" >> "$GITHUB_OUTPUT"
           fi
+
   build:
     needs:
       - generate-matrix

.github/workflows/windows.yml

@@ -79,12 +79,13 @@ jobs:
             echo "any_builds=false" >> $GITHUB_OUTPUT
           fi
 
-
       - name: Check if the `pythonbuild` crate changed
         id: check-pythonbuild
-        shell: bash
+        env:
+          BASE_REF: ${{ github.event.pull_request.base.ref || 'main' }}
         run: |
-          if git diff --quiet ${{ github.event.pull_request.base.sha || 'origin/main' }}...HEAD -- ':src/*.rs'; then
+          merge_base=$(git merge-base HEAD "origin/${BASE_REF}")
+          if git diff --quiet "${merge_base}...HEAD" -- ':src/*.rs'; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else
               echo "changed=true" >> "$GITHUB_OUTPUT"