fix: use exact ref check for nightly-testing branch existence

This PR switches from `git ls-remote --heads origin nightly-testing` (which
does substring pattern matching and could match e.g.
`refs/heads/foo/nightly-testing`) to `git show-ref --verify --quiet` on the
local tracking ref after fetch, which is an exact match.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>