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

.github/workflows/check-build.yml

@@ -6,8 +6,8 @@ on:
       - master
       - develop
 jobs:
-  check:
-    name: Flake Check (x86_64 only)
+  check-emacs:
+    name: Flake Check emacs (x86_64 only)
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
@@ -34,9 +34,43 @@ jobs:
           nix-store --import < nix-store.dump || true
           rm nix-store.dump
         fi
-    - run: |
+    - name: Run checks in emacs
+      run: |
         nix build .#checks.x86_64-linux.init-example-el
     - name: Export /nix/store contents
       if: ${{ !steps.cache-nix-store.outputs.cache-hit }}
       run: |
         nix-store --export $(nix-store -qR /nix/store/*-doom-emacs) > nix-store.dump
+  check-emacsGit:
+    name: Flake Check emacsGit (x86_64 only)
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+      with:
+          # Nix Flakes doesn't work on shallow clones
+          fetch-depth: 0
+    - uses: cachix/install-nix-action@v17
+      with:
+        name: nix-community
+    - name: Retrieve /nix/store archive
+      uses: actions/cache@v3
+      id: cache-nix-store-emacsGit
+      with:
+        path: nix-store.dump
+        key: nix-store-emacsGit-${{ hashFiles('flake.*') }}
+        restore-keys: |
+          nix-store-emacsGit-
+    - name: Import /nix/store contents
+      if: ${{ steps.cache-nix-store-emacsGit.outputs.cache-hit }}
+      run: |
+        if [[ -f nix-store.dump ]]; then
+          nix-store --import < nix-store.dump || true
+          rm nix-store.dump
+        fi
+    - name: Run checks in emacsGit
+      run: |
+        nix build .#checks.x86_64-linux.init-example-el-emacsGit
+    - name: Export /nix/store contents
+      if: ${{ !steps.cache-nix-store-emacsGit.outputs.cache-hit }}
+      run: |
+        nix-store --export $(nix-store -qR /nix/store/*-doom-emacs) > nix-store.dump