Add git-instafix

As seen on
https://www.youtube.com/live/ik_d0dWYQgQ?si=XSNR00ovIgSu7Ent&t=6510,
`git-instafix` is a tool for quickly fix up an old commit using your
currently-staged changes.

https://github.com/quodlibetor/git-instafix

lib/shared/modules/git.nix

@@ -1,3 +1,5 @@
+{ pkgs, ... }:
+
 {
   home.file.".gitmessage".text = ''
 
@@ -136,4 +138,6 @@
       };
     };
   };
+
+  home.packages = with pkgs; [ git-instafix ];
 }