Merge branch 'nixpkgs-git-revision-fix'

Addresses #17218 in a better way in that it doesn't create a
".git-revision" file on every nixos-rebuild, because we already have
".git" available. Even if we don't nixos-rebuild can't create the
"git-revision" file.

Tested via:

nix-build -E '(import ./nixos/tests/make-test.nix {
  name = "foo";
  machine = {};
  testScript = "startAll; $machine->execute(\"nixos-version >&2\");";
})'

Closes: #17610
Acked-by: @bennofs

nixos/modules/installer/tools/get-version-suffix

@@ -17,6 +17,6 @@ getVersion() {
 if nixpkgs=$(nix-instantiate --find-file nixpkgs "$@"); then
     getVersion $nixpkgs
     if [ -n "$rev" ]; then
-        echo "$rev"
+        echo ".git.$rev"
     fi
 fi

nixos/modules/installer/tools/nixos-rebuild.sh

@@ -311,10 +311,9 @@ fi
 # nixos-version shows something useful).
 if [ -n "$canRun" ]; then
     if nixpkgs=$(nix-instantiate --find-file nixpkgs "${extraBuildFlags[@]}"); then
-        revision=$($SHELL $nixpkgs/nixos/modules/installer/tools/get-git-revision "${extraBuildFlags[@]}" || true)
-        if [ -n "$revision" ]; then
-            echo -n ".git.$revision" > "$nixpkgs/.version-suffix" || true
-            echo -n "$revision"  > "$nixpkgs/.git-revision" || true
+        suffix=$($SHELL $nixpkgs/nixos/modules/installer/tools/get-version-suffix "${extraBuildFlags[@]}" || true)
+        if [ -n "$suffix" ]; then
+            echo -n "$suffix" > "$nixpkgs/.version-suffix" || true
         fi
     fi
 fi

nixos/modules/misc/version.nix

@@ -63,7 +63,9 @@ in
     nixosRevision = mkOption {
       internal = true;
       type = types.str;
-      default = if pathExists revisionFile then fileContents revisionFile else "master";
+      default = if pathIsDirectory gitRepo then commitIdFromGitRepo gitRepo
+                else if pathExists revisionFile then fileContents revisionFile
+                else "master";
       description = "The Git revision from which this NixOS configuration was built.";
     };