perf(git): fetch only the refs a comparison needs

Git.__init__() ran `git fetch --all` on every invocation, pulling every remote
branch and tag before changed-file detection even began. Resolve commit and
branch metadata locally instead, and for pull-request comparisons prefer refs
already present in the checkout, fetching a single base or head ref only when it
is missing.

Also recognises Buildkite's native BUILDKITE_COMMIT, BUILDKITE_BRANCH,
BUILDKITE_PULL_REQUEST and BUILDKITE_PULL_REQUEST_BASE_BRANCH so Buildkite jobs
can calculate a complete base-to-head changed-file range without mapping their
environment onto GitHub Actions variable names. Buildkite is checked before
GitHub because some pipelines deliberately export GitHub-compatible variables.

Adds INFO durations for Git initialisation, changed-file detection and each
fetch, including the ref requested and why. Existing GitHub Actions, GitLab CI,
Bitbucket Pipelines and local behaviour is preserved; tests cover local-ref
preference, absence of an unconditional fetch, the targeted-fetch fallback, all
four CI providers, and non-PR and detached-HEAD execution.

Ref: CE-379

Author: lelia