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 <vbatts@hashbangbash.com>