home: git: use 'zdiff3' merge conflict markers

See GitHub's presentation about this feature [1].

[1]: https://github.blog/2022-01-24-highlights-from-git-2-35/

home/git/default.nix

@@ -120,6 +120,10 @@ in
         defaultBranch = "main";
       };
 
+      merge = {
+        conflictStyle = "zdiff3";
+      };
+
       pull = {
         # Avoid useless merge commits
         rebase = true;