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_convert_gogit.go

@@ -47,6 +47,12 @@ func convertPGPSignature(c *object.Commit) *CommitGPGSignature {
 		return nil
 	}
 
+	if c.Encoding != "" && c.Encoding != "UTF-8" {
+		if _, err = fmt.Fprintf(&w, "\nencoding %s\n", c.Encoding); err != nil {
+			return nil
+		}
+	}
+
 	if _, err = fmt.Fprintf(&w, "\n\n%s", c.Message); err != nil {
 		return nil
 	}