[doc] Fix no-new-rst lint check crashing on stacked PRs

[elliot-barn]

Why are these changes needed?

The doc_no_new_rst lint check (doc/test_no_new_rst.py) fetches the base branch with git fetch origin <base> and then diffs against origin/<base>:

subprocess.check_call(["git", "fetch", "-q", remote, base])
out = subprocess.check_output([
    "git", "diff", "--name-only", "--diff-filter=A",
    f"{remote}/{base}...{commit}", "--",
])

git fetch origin <base> populates FETCH_HEAD but does not create a remote-tracking ref refs/remotes/origin/<base>. So origin/<base> only resolves for branches already present in the clone (e.g. master). For PRs whose base is another feature branch (stacked PRs), origin/<base> is absent and the diff aborts with fatal: bad revision (git exit 128) — failing the check before it ever evaluates whether any .rst files were added.

This fix diffs against FETCH_HEAD — which the preceding git fetch always populates with the base tip — instead of origin/<base>. Behavior is unchanged for master-based PRs (the common case); the check is fixed for stacked PRs.

The change is pure git mechanics with no behavioral change to what the check enforces (still flags newly-added .rst files under doc/source/).

Related issue number

N/A

Checks

• I've signed off every commit (by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[gemini-code-assist]

Code Review

This pull request updates the git diff command in doc/test_no_new_rst.py to compare against FETCH_HEAD instead of {remote}/{base}. There are no review comments, and I have no feedback to provide.

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