https://marketplace.visualstudio.com/items?itemName=mhutchie.git-graph
> File Encoding: The character set encoding used when retrieving a specific version of repository files (e.g. in the Diff View). A list of all supported encodings can be found here.
しらんけどできると書いてるから調べてみたら

本来gitattributes使ってUTF-8に寄せるのが筋だろうけど