fix(ci): diff pre-commit changed files against the live base branch tip

[rusackas]

SUMMARY

The pre-commit workflow's changed-files detection diffs against github.event.pull_request.base.sha, which GitHub freezes at PR creation. On a long-lived PR the diff therefore runs from the original branch point to the merge ref (which contains current master), sweeping in all of master's churn since the branch was cut.

This has two failure modes, both observed on #41127 (branched before the antd v6 upgrade and the i18n catalog canonicalization landed):

1. Silent over-linting: pre-commit runs on hundreds/thousands of files the PR never touched.
2. Hard failure: once the file list crosses the kernel's per-env-var limit (MAX_ARG_STRLEN, 128KiB), the runner cannot even start bash for the step — it dies with An error occurred trying to start process '/usr/bin/bash' ... Argument list too long and no hook output. Re-runs replay the same frozen payload, so the job can never go green again; the only workaround is closing and reopening the PR.

The fix:

• For pull_request events, diff against HEAD^1 instead. The checkout is the PR merge commit, so its first parent is the current base-branch tip, and the three-dot diff yields exactly the PR's own files regardless of how old the branch is. The existing fail-closed guard (fall back to --all-files when the base can't be resolved) is kept and covers the no-merge-commit edge.
• Cap the computed list at 100KB (safely under the 128KiB limit): a PR that legitimately touches thousands of files now degrades to an --all-files sweep instead of an unstartable step.

push/schedule behavior is unchanged.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A (CI behavior)

TESTING INSTRUCTIONS

• check-yaml and zizmor pre-commit hooks pass on the workflow file.
• Behavior is observable on any PR whose branch point predates a large master change: previously the changed-files list contained all of master's intervening churn; with this change it contains only the PR's files. The failing case is reproducible on fix(sqllab): apply SQL_QUERY_MUTATOR in SQL Lab when MUTATE_AFTER_SPLIT is set #41127 prior to its close/reopen.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

🤖 Generated with Claude Code

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[hainenber]

IMO, if there are sufficient files getting changed in a long-live PR, like in huge migrations, we should just run the whole precommit against the codebase once a threshold reached. It'd make the logic much more simpler.