Include encoding in signature payload (#30174)

Fixes #30119

Include the encoding in the signature payload.

before

![grafik](https://github.com/go-gitea/gitea/assets/1666336/01ab94a3-8af5-4d6f-be73-a10b65a15421)

after

![grafik](https://github.com/go-gitea/gitea/assets/1666336/3a37d438-c70d-4d69-b178-d170e74aa683)

modules/git/commit_reader.go

@@ -84,6 +84,8 @@ readLoop:
 				commit.Committer = &Signature{}
 				commit.Committer.Decode(data)
 				_, _ = payloadSB.Write(line)
+			case "encoding":
+				_, _ = payloadSB.Write(line)
 			case "gpgsig":
 				fallthrough
 			case "gpgsig-sha256": // FIXME: no intertop, so only 1 exists at present.