Use merge-base for git diff in get-changed-files.sh (#57487)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Sarah Schneider <sarahs@users.noreply.github.com>

Author: 3 people

.github/actions/get-changed-files/get-changed-files.sh

@@ -18,8 +18,9 @@ git fetch --depth=1 origin main
 git fetch --depth=1 origin ${INPUT_HEAD:-HEAD}
 
 # Get diff with status information
+# Find the merge-base (common ancestor) instead of using origin/main directly
 echo "__ running git diff with status __"
-DIFF_OUTPUT=$(git diff --name-status origin/main origin/${INPUT_HEAD:-HEAD})
+DIFF_OUTPUT=$(git diff --name-status origin/main...origin/${INPUT_HEAD:-HEAD})
 
 # Function to extract files by pattern from diff output
 extract_files() {