changelog/v0.21: add dag stat additions

docs/changelogs/v0.21.md

@@ -11,6 +11,7 @@
   - [`client/rpc` migration of `go-ipfs-http-client`](#clientrpc-migration-of-go-ipfs-http-client)
   - [Gateway: DAG-CBOR/-JSON previews and improved error pages](#gateway-dag-cbor-json-previews-and-improved-error-pages)
   - [Gateway: subdomain redirects are now `text/html`](#gateway-subdomain-redirects-are-now-texthtml)
+  - [`ipfs dag stat` deduping statistics](#ipfs-dag-stat-deduping-statistics)
 - [📝 Changelog](#-changelog)
 - [👨‍👩‍👧‍👦 Contributors](#-contributors)
 
@@ -102,6 +103,27 @@ $ curl "https://subdomain-gw.example.net/ipfs/${cid}/"
 
 Rationale can be found in [kubo#9913](https://github.com/ipfs/kubo/pull/9913).
 
+#### `ipfs dag stat` deduping statistics
+
+`ipfs dat stat` now accept multiple CIDs and will dump advanced statistics
+on the number of shared blocks and size of each CID.
+
+```console
+$ ipfs dag stat --progress=false QmfXuRxzyVy5H2LssLgtXrKCrNvDY8UBvMp2aoW8LS8AYA QmfZDyu2UFfUhL4VdHaw7Hofivmn5D4DdQj38Lwo86RsnB
+
+CID                                           	Blocks         	Size
+QmfXuRxzyVy5H2LssLgtXrKCrNvDY8UBvMp2aoW8LS8AYA	3              	2151
+QmfZDyu2UFfUhL4VdHaw7Hofivmn5D4DdQj38Lwo86RsnB	4              	3223
+
+Summary
+Total Size: 3326
+Unique Blocks: 5
+Shared Size: 2048
+Ratio: 1.615755
+```
+
+`ipfs --enc=json dag stat`'s keys are a non breaking change, new keys have been added but old keys with previous sementics are still here.
+
 ### 📝 Changelog
 
 ### 👨‍👩‍👧‍👦 Contributors