fix: strengthen validation in conflict resolver to prevent bad merge commits (#26754)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.github/workflows/devin-conflict-resolver.yml

@@ -316,6 +316,8 @@ jobs:
             Base Branch: ${pr.base_ref}
             ${pr.is_fork ? `Fork Repository: ${pr.head_repo_owner}/${pr.head_repo_name}` : ''}
 
+            IMPORTANT WARNING: If your merge commit shows more than 50 files changed, DO NOT PUSH. This indicates the merge was done incorrectly. Abort and leave a comment instead.
+
             Your tasks:
             ${forkInstructions}
 
@@ -327,13 +329,32 @@ jobs:
                - If unsure about a conflict, prefer keeping both changes where possible
             2. Test that the code still works after resolving conflicts (run lint/type checks).
             3. Commit the merge resolution with a clear commit message.
-            4. CRITICAL VALIDATION BEFORE PUSHING: You MUST validate your merge commit before pushing:
-               - Run: git diff --stat HEAD^1...HEAD (shows changes relative to PR branch parent)
-               - Run: git diff --stat HEAD^2...HEAD (shows changes relative to base branch parent)
-               - The changes relative to the base branch parent (HEAD^2) should be SIMILAR to the original PR changes, NOT include all the new commits from ${pr.base_ref}
-               - If your merge commit shows hundreds of files changed or includes changes that were already in ${pr.base_ref}, STOP IMMEDIATELY
-               - A proper merge commit only contains conflict resolutions, not reproduced changes from the target branch
-               - If validation fails: DO NOT PUSH. Log the error, explain what went wrong, and abort the task.
+            4. CRITICAL VALIDATION BEFORE PUSHING - YOU MUST RUN THESE EXACT COMMANDS:
+               
+               Step A: Count files changed in your merge commit:
+               \`\`\`
+               FILE_COUNT=$(git diff --name-only HEAD^1...HEAD | wc -l)
+               echo "Files changed in merge commit: $FILE_COUNT"
+               \`\`\`
+               
+               Step B: If FILE_COUNT > 50, STOP IMMEDIATELY. DO NOT PUSH.
+               - A proper merge commit should only contain conflict resolutions
+               - If you see 50+ files changed, the merge was done INCORRECTLY
+               - This usually means you accidentally reproduced changes from ${pr.base_ref} instead of just resolving conflicts
+               
+               Step C: Compare your merge to the original PR:
+               \`\`\`
+               git diff --stat HEAD^1...HEAD
+               \`\`\`
+               - This should show ONLY the files that had conflicts, plus any files from ${pr.base_ref} that were merged in
+               - If you see hundreds of files or files unrelated to the original PR conflicts, STOP
+               
+               Step D: If validation fails:
+               - DO NOT PUSH under any circumstances
+               - DO NOT attempt to fix it
+               - Leave a comment on the PR explaining that the merge validation failed
+               - ABORT THE TASK IMMEDIATELY
+               - It is better to leave the PR with conflicts than to push a broken merge commit that pollutes git history
             5. Push the resolved changes to the PR branch only after validation passes.
             6. After successfully pushing the resolved changes, remove the \`devin-conflict-resolution\` label from the PR using the GitHub API.
 
@@ -345,7 +366,8 @@ jobs:
             4. If a conflict seems too complex or risky to resolve automatically, explain the situation in a PR comment instead.
             5. Never ask for user confirmation. Never wait for user messages.
             6. CRITICAL: If this is a fork PR and you encounter ANY error when pushing (permission denied, authentication failure, etc.), you MUST fail the task immediately. Do NOT attempt to push to a new branch in the main ${owner}/${repo} repository as a workaround. Simply report the error and stop.
-            7. CRITICAL: Never reproduce or recreate changes from the target branch. Your merge commit should ONLY contain conflict resolutions. If you find yourself manually copying file contents from ${pr.base_ref} or creating changes that mirror what's already in ${pr.base_ref}, you are doing it wrong. Use git's merge functionality properly - it handles bringing in changes automatically.`;
+            7. CRITICAL: Never reproduce or recreate changes from the target branch. Your merge commit should ONLY contain conflict resolutions. If you find yourself manually copying file contents from ${pr.base_ref} or creating changes that mirror what's already in ${pr.base_ref}, you are doing it wrong. Use git's merge functionality properly - it handles bringing in changes automatically.
+            8. CRITICAL: If your merge commit shows a large number of files changed (more than 50), DO NOT PUSH UNDER ANY CIRCUMSTANCES. This is a sign that the merge was done incorrectly. Abort the task and leave a comment explaining the issue. A bad merge commit in the git history is worse than leaving the PR with conflicts.`;
 
             try {
               let sessionUrl;