gomtree: `-list-used` can output JSON
gomtree: `-list-used` can output JSON

Piggybacking on `-result-format`:

```bash
$ tar c .git/ | gomtree -c -T - > git.mtree
$ gomtree -result-format=json -list-used -f ./git.mtree
{
  "./git.mtree": [
    "type",
    "mode",
    "uid",
    "gid",
    "tar_time",
    "size"
  ]
}
$ tar c .git/ | gomtree -c -T - -K sha512digest > git.mtree
$ gomtree -result-format=json -list-used -f ./git.mtree
{
  "./git.mtree": [
    "type",
    "mode",
    "uid",
    "gid",
    "tar_time",
    "size",
    "sha512digest"
  ]
}
```

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

vbatts/go-mtree,vbatts/go-mtree