checkUpdates() ran `git rev-list HEAD..origin/<branch> --count`
unconditionally in the parallel probe batch, even on the shallow +
no-merge-base path where resolveBehindCount() ignores the result and
falls back to a SHA compare. In the #51922 failure mode that count walks
the entire remote ancestry (thousands of commits), so the work was pure
latency on every update check for the exact case the fix targets.
Split the probes into two phases: resolve --is-shallow-repository and
merge-base first, then run rev-list --count only when shouldCountCommits
says the number is meaningful (full clone, or shallow-with-merge-base).
The shallow/no-merge-base SHA fallback is preserved unchanged.
