`br debug decode` is useless when the backupmeta file got split.

[kennytm]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Generate a full backup locally br backup full -s '/tmp/testbr
2. Decode the backupmeta file br debug decode -s '/tmp/testbr

2. What did you expect to see? (Required)

The entire content of the backupmeta can be readable in JSON

3. What did you see instead (Required)

We can only see the main backupmeta.json is referring to the split-off files:

{
  "ddls": [],
  "cluster_id": 7418281098110393269,
  "cluster_version": "8.1.0",
  "br_version": "«snip»",
  "version": 1,
  "file_index": {
    "meta_files": [
      {
        "name": "backupmeta.datafile.000000001",
        "sha256": "2XIqo5osORmCTs0fWf0T59xYI98i6nZ/5IdqYwNQmF0=",
        "size": 4623
      }
    ]
  },
  "end_version": 452789007127937031,
  "schema_index": {
    "meta_files": [
      {
        "name": "backupmeta.schema.000000002",
        "sha256": "p2CbZh4isJ7lRdFWDoa/rQEYsV+0jr/v2KzX0JT2GeE=",
        "size": 290142
      }
    ]
  },
  "new_collations_enabled": "True",
  "backup_size": 401424
}

and the split-off files are not decoded into JSON:

$ ls -aF1 /tmp/testbr
./
../
1/
backup.lock
backupmeta
backupmeta.datafile.000000001
backupmeta.json
backupmeta.schema.000000002
checkpoints/

4. What is your TiDB version? (Required)

checked on v8.1.0, master.