nixos/modules/misc/version.nix: check that .git is a directory

That's not the case for git submodules
Fixes #15928

lib/sources.nix

@@ -4,6 +4,11 @@ let lib = import ./default.nix; in
 
 rec {
 
+  # Returns the type of a path: regular (for file), symlink, or directory
+  pathType = p: with builtins; getAttr (baseNameOf p) (readDir (dirOf p));
+
+  # Returns true if the path exists and is a directory, false otherwise
+  pathIsDirectory = p: if builtins.pathExists p then (pathType p) == "directory" else false;
 
   # Bring in a path as a source, filtering out all Subversion and CVS
   # directories, as well as backup files (*~).
@@ -29,6 +34,7 @@ rec {
       in type == "directory" || lib.any (ext: lib.hasSuffix ext base) exts;
     in builtins.filterSource filter path;
 
+
   # Get the commit id of a git repo
   # Example: commitIdFromGitRepo <nixpkgs/.git>
   commitIdFromGitRepo =

nixos/modules/misc/version.nix

@@ -104,8 +104,8 @@ in
       # changing them would not rebuild the manual
       nixosLabel   = mkDefault (maybeEnv "NIXOS_LABEL" cfg.nixosVersion);
       nixosVersion = mkDefault (maybeEnv "NIXOS_VERSION" (cfg.nixosRelease + cfg.nixosVersionSuffix));
-      nixosRevision      = mkIf (pathExists gitRepo) (mkDefault            gitCommitId);
-      nixosVersionSuffix = mkIf (pathExists gitRepo) (mkDefault (".git." + gitCommitId));
+      nixosRevision      = mkIf (pathIsDirectory gitRepo) (mkDefault            gitCommitId);
+      nixosVersionSuffix = mkIf (pathIsDirectory gitRepo) (mkDefault (".git." + gitCommitId));
 
       # Note: code names must only increase in alphabetical order.
       nixosCodeName = "Flounder";