Fetch pull request commits for incremental spellcheck

[mcagnion]

Description of the problem being solved:

The shallow checkout used by the spellcheck workflow can omit the pull request's head commit. When the incremental diff cannot read that object, spellcheck falls back to scanning the whole repository and reports spelling errors in unchanged files.

Fetch the event's base and head commits at depth 1 before running spellcheck. This supplies the objects needed for the diff without fetching the full repository history. Other workflow events are unchanged.

Steps taken to verify a working solution:

• Reproduced the missing-head-object failure in a shallow clone.
• Fetched the two event commits and verified that the same diff command returned only the changed files.
• Validated the workflow with actionlint.