nixos-rebuild-ng: handle subflakes correctly

Tested with a simple flake setup:

/tmp/zen-mcnulty-jHbPy1 main*
% /nix/store/9a2w8pb4zxz084hmjbfcmgpswan1kz51-nixos-rebuild-ng-0.0.0/bin/nixos-rebuild-ng repl --flake .#nixos
Nix 2.28.3
nix-repl> config.networking.hostName
"nixos"

/tmp/zen-mcnulty-jHbPy1 main*

% cd subflake
 configuration.nix   flake.nix   hardware-configuration.nix   tmp/

/tmp/zen-mcnulty-jHbPy1/subflake main*
% /nix/store/9a2w8pb4zxz084hmjbfcmgpswan1kz51-nixos-rebuild-ng-0.0.0/bin/nixos-rebuild-ng repl --flake .#nixos
nix-repl> config.networking.hostName
"foo"

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/models.py

@@ -61,19 +61,39 @@ class BuildAttr:
         return cls(Path(file or "default.nix"), attr)
 
 
-def discover_git(location: Path) -> str | None:
+def discover_git(location: Path) -> Path | None:
+    """
+    Discover the current git repository in the given location.
+    """
     current = location.resolve()
     previous = None
 
     while current.is_dir() and current != previous:
         dotgit = current / ".git"
         if dotgit.is_dir():
-            return str(current)
+            return current
         elif dotgit.is_file():  # this is a worktree
             with dotgit.open() as f:
                 dotgit_content = f.read().strip()
                 if dotgit_content.startswith("gitdir: "):
-                    return dotgit_content.split("gitdir: ")[1]
+                    return Path(dotgit_content.split("gitdir: ")[1])
+        previous = current
+        current = current.parent
+
+    return None
+
+
+def discover_closest_flake(location: Path) -> Path | None:
+    """
+    Discover the closest flake.nix file starting from the given location upwards.
+    """
+    current = location.resolve()
+    previous = None
+
+    while current.is_dir() and current != previous:
+        flake_file = current / "flake.nix"
+        if flake_file.is_file():
+            return current
         previous = current
         current = current.parent
 
@@ -110,7 +130,15 @@ class Flake:
             path = Path(path_str)
             git_repo = discover_git(path)
             if git_repo is not None:
-                return cls(f"git+file://{git_repo}", nixos_attr)
+                url = f"git+file://{git_repo}"
+                flake_path = discover_closest_flake(path)
+                if (
+                    flake_path is not None
+                    and flake_path != git_repo
+                    and flake_path.is_relative_to(git_repo)
+                ):
+                    url += f"?dir={flake_path.relative_to(git_repo)}"
+                return cls(url, nixos_attr)
             return cls(path, nixos_attr)
 
     @classmethod