Option to fetch the base branch during checkout, not just for pipeline upload's if_changed diff

## Is your feature request related to a problem? Please describe.

A job's checkout fetches `refs/pull/N/head` plus the commit (or the commit alone for a branch build). None of those update a branch's remote-tracking ref, so `refs/remotes/origin/` in a reused checkout directory holds whatever an earlier build left there — stale by an unbounded amount, with nothing in the log to say so.

`--fetch-diff-base` / `BUILDKITE_FETCH_DIFF_BASE` solves this for `buildkite-agent pipeline upload`'s own `if_changed` diff. Anything else in the job that asks what changed — a pipeline generator, a test selector, a script running `git merge-base HEAD origin/main` — still reads the stale ref. A stale base only ever widens a diff, so nothing fails and nobody notices.

The fix today is per step: a plugin, or an explicit `git fetch` in the command. Every new step has to remember it, and a step that forgot looks identical to one that didn't.

## Describe the solution you'd like

An agent-level option, off by default, that fetches the base branch during checkout so every job in the build starts from its current tip: `--git-fetch-base-branch` / `BUILDKITE_GIT_FETCH_BASE_BRANCH`, on `agent start` and `bootstrap`.

## Describe alternatives you've considered

- `BUILDKITE_GIT_FETCH_FLAGS`: flags land before `--`, so it cannot add a refspec.
- `BUILDKITE_REFSPEC`: replaces the PR fetch rather than adding to it.
- `--fetch-diff-base`: scoped to `pipeline upload`, and only to the diff it computes itself.

## Affiliation (optional)

Altana.

## Additional context

Happy to send a PR — I have one ready.

Start at the agent start and bootstrap checkout paths, then read the existing --fetch-diff-base handling and Git refspec construction. Trace how CLI options and BUILDKITE_* environment variables are wired into checkout. Done means an off-by-default base-branch fetch is available through both entry points and updates the base remote-tracking ref for reused checkouts.

Written by the indexing model from the issue text.