jenkins: fetch refs before test commit rebase

[trivikr]

Fixes: #4345

Refresh the PR ref and REBASE_ONTO in node-test-commit-pre.sh before
checking out _jenkins_local_branch and running git rebase.

When Jenkins has a shallow copy where REBASE_ONTO exists locally without
its parents, git rebase can treat nearly the entire Node.js history as
commits to replay. That caused failures like:

Rebasing (1/46550)
Auto-merging Makefile
CONFLICT (add/add): Merge conflict in Makefile

---

The script now fetches the refs it is about to use and unshallows the repository when needed.

Validation

• bash -n jenkins/scripts/node-test-commit-pre.sh
• git diff --check
• Reproduced the bad shallow range locally: before=46899
• Confirmed the added fetch behavior reduces it to the expected range: after=1

---

Assisted-by: openai:gpt-5.5

[sxa]

This is probably ok but it's so hot in the room I'm working I don't trust myself to do a formal review at the moment ...
I've run another job and that seems to be running ok so I'll keep an eye on subsequent runs.

[targos]

This probably works, but I don't think it's ok. We have never had these failures before so I don't think we should hide them by changing a script that has been working for years.

[sxa]

Intriguingly it's shown up on a second debian12-x64 system overnight too 🤔

I sort of agree with targos but it's notable that it has happened on a second machine of the same OS in the same week. I'll keep an eye on it and do some investigation outside this issue, regardless of what happens with this PR in case there's an underlying cause that needs to be addressed.