feat(git): use 'nvimdiff' as difftool

dots/.gitconfig

@@ -80,6 +80,12 @@
   conflictstyle = diff3
 [diff]
   colorMoved = zebra
+  tool = nvimdiff
+[difftool]
+  prompt = false
+[mergetool]
+  prompt = false
+  keepBackup = false
 [commit]
   gpgsign = false
 

home/modules/git/default.nix

@@ -14,6 +14,12 @@
   };
 
   config = lib.mkIf config.git.enable {
+    assertions = [
+      {
+        assertion = config.nvim.enable;
+        message = "git module requires 'nvim' module (`nvimdiff`)";
+      }
+    ];
     programs.git.enable = true;
     home.file = {
       ".gitconfig".source = dotsPath + "/.gitconfig";