ci(jscpd): merge target branch in jscpd to avoid false negatives. (aws#6572)

## Problem
- Follow up to
aws#6564 (review).

## Solution
- It appears that there is an undocumented "feature" that GHA don't run
when there is a merge conflict. See
[here](https://github.com/orgs/community/discussions/11265)
- This means we don't have to handle the failure case where a merge
fails.
- Add fake config identity to mitigate this error: 
<img width="913" alt="image"
src="https://github.com/user-attachments/assets/cd426ec7-e1ca-4d13-a3b1-3985b5593c07"
/>


## Notes
Going to let this sit and make sure it works as changes are merged into
master.


---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

---------

Co-authored-by: Justin M. Keyes <[email protected]>

Author: 2 people

.github/workflows/filterDuplicates.js

@@ -111,9 +111,6 @@ async function run() {
     console.log('%s duplicates found', filteredDuplicates.length)
     if (filteredDuplicates.length > 0) {
         console.log(formatDuplicates(filteredDuplicates, commitHash, repoName))
-        console.log(
-            '* Hint: if these duplicates appear unrelated to the changes, rebase onto the latest target branch.'
-        )
         process.exit(1)
     }
 }

.github/workflows/node.js.yml

@@ -84,6 +84,17 @@ jobs:
                   git remote add forkUpstream https://github.com/$REPO_NAME  # URL of the fork
                   git fetch forkUpstream  # Fetch fork
 
+            - name: Merge in target branch to avoid false negatives.
+              env:
+                  TARGET_BRANCH: ${{ github.event.pull_request.base.ref }}
+              # Note: "git merge" should always succeed here, because GHA won't
+              # start the job if there are merge conflicts. https://github.com/orgs/community/discussions/11265
+              # Also, because `git merge` makes a commit, we need to establish an identity to avoid 'Committer identity unknown' error
+              run: |
+                  git config --global user.name "aws-toolkit-automation"
+                  git config --global user.email "<>"
+                  git merge origin/$TARGET_BRANCH
+
             - name: Compute git diff
               env:
                   CURRENT_BRANCH: ${{ github.head_ref }}