nixvim: add diffview, gitsigns, which-key. re-enable treesitter. document some keymaps

modules/home/nixvim/plugins/gitsigns.nix

@@ -0,0 +1,48 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.programs.nixvim;
+  plugin = cfg.plugins.fugitive;
+
+  inherit (lib) mkDefault mkIf;
+in
+{
+  config = {
+    programs.nixvim = {
+      plugins.gitsigns = {
+        enable = mkDefault true;
+        settings = {
+          current_line_blame = mkDefault false;
+          current_line_blame_opts = mkDefault {
+            virt_text = mkDefault true;
+            virt_text_pos = mkDefault "eol";
+          };
+          signcolumn = mkDefault true;
+          signs = {
+            add = {
+              text = mkDefault "+";
+            };
+            change = {
+              text = mkDefault "│";
+            };
+            changedelete = {
+              text = mkDefault "~";
+            };
+            delete = {
+              text = mkDefault "-";
+            };
+            topdelete = {
+              text = mkDefault "-";
+            };
+            untracked = {
+              text = mkDefault "?";
+            };
+          };
+          watch_gitdir = {
+            follow_files = mkDefault true;
+          };
+        };
+      };
+    };
+  };
+}