Use --init-directory in Emacs 29+ (#191)

* Use `--init-directory` in Emacs 29+

This will allow a few advantages:
- We can load `early-init.el` file properly, fixing Flash Of Unstyled
Contents (FOUC) issues and improving the performance slightly
- Eventually we can drop `default.el` file loading in Home-Manager
module. Not done here since this would complicate the code without
necessity, however this can be done once Emacs 29+ is default

* Fix build in Emacs <29

* Add #checks.<arch>.init-example-el-emacsGit

* Add .#checks.x86_64-linux.init-example-el-emacsGit to CI

* Fix review issues

* Move config files to share/emacs.d

* Separate tests in two steps

* Add TODOS

* Simplify emacs-dir generation

flake.nix

@@ -87,7 +87,7 @@
     flake-compat.flake = false;
   };
 
-  outputs = { self, nixpkgs, flake-utils, ... }@inputs:
+  outputs = { self, nixpkgs, flake-utils, emacs-overlay, ... }@inputs:
     let inherit (flake-utils.lib) eachDefaultSystem eachSystem;
     in eachDefaultSystem (system:
       let pkgs = import nixpkgs { inherit system; };
@@ -100,12 +100,27 @@
           pkgs.callPackage self
           (args // { dependencyOverrides = (inputs // dependencyOverrides); });
       }) // eachSystem [ "x86_64-linux" "aarch64-darwin" ] (system: {
-        checks = {
-          init-example-el = self.outputs.package.${system} {
-            doomPrivateDir = ./test/doom.d;
-            dependencyOverrides = inputs;
-          };
-        };
+        checks =
+          let
+            pkgs = import nixpkgs {
+              inherit system;
+              # we are not using emacs-overlay's flake.nix here,
+              # to avoid unnecessary inputs to be added to flake.lock;
+              # this means we need to import the overlay in a hack-ish way
+              overlays = [ (import emacs-overlay) ];
+            };
+          in
+            {
+              init-example-el = self.outputs.package.${system} {
+                doomPrivateDir = ./test/doom.d;
+                dependencyOverrides = inputs;
+              };
+              init-example-el-emacsGit = self.outputs.package.${system} {
+                doomPrivateDir = ./test/doom.d;
+                dependencyOverrides = inputs;
+                emacsPackages = with pkgs; emacsPackagesFor emacsGit;
+              };
+            };
       }) // {
         hmModule = import ./modules/home-manager.nix inputs;
       };