Tools: bucket rewrite tool doesn't work with dry run and multiple block IDs

[yeya24]

Thanos, Prometheus and Golang version used: main

When using bucket rewrite tool with dry run mode and specifying multiple blocks, it only processes the first block and the process exits.

From the code https://github.com/thanos-io/thanos/blob/main/cmd/thanos/tools_bucket.go#L1137, it returns nil when dry run mode is enabled. It should continue execution in this case.

[4molybdenum2]

@yeya24 made a PR. Can you check?

[yeya24]

Just to double check. For each specified block, rewrite tool will rewrite it separately. Is this intended or should it rewrite all specified blocks into a larger one?
cc @bwplotka

[yeya24]

Close this for now.