Add --target-max-ref-updates flag; quiet ref-only batch progress

Plumb a per-target ref-update batch size through the Pusher (MaxRefUpdates), the syncer config, the unstable Options, and the replicate/sync/plan/bootstrap commands as --target-max-ref-updates. Zero keeps the env-or-default limit (GITSYNC_MAX_REF_UPDATES_PER_PUSH or 5000); a positive value overrides it — raise it for entire-server targets (up to 25k), lower it for stricter providers. No global mutable state: the value rides on the Pusher.

Also stop spewing a bare "target:" sideband line per ref-only follow-up batch: those carry no useful progress, so push them with progress suppressed and, when verbose, emit one concise "pushed ref-update batch N/M (K refs)" line per batch instead.