vscode-extensions.mhutchie.git-graph: correct license

Versions ≤1.4.6 are licensed under the MIT License, but versions ≥1.5.0
have a derived unfree license that explicitly prohibits redistribution:

> Permission is hereby granted, free of charge, to any person obtaining a copy
> of this software and associated documentation files (the "Software"), to use,
> copy, modify, merge, and to permit persons to whom the Software is
> furnished to do so, subject to the following conditions:
>
> Permission is NOT GRANTED to publish, distribute, sublicense, and/or sell
> derivative works of the Software.
>
> […]

pkgs/applications/editors/vscode/extensions/default.nix

@@ -3165,7 +3165,7 @@ let
           hash = "sha256-sHeaMMr5hmQ0kAFZxxMiRk6f0mfjkg2XMnA4Gf+DHwA=";
         };
         meta = {
-          license = lib.licenses.mit;
+          license = lib.licenses.unfree;
         };
       };