lib.fileset.gitTracked: Improve error when passing files

lib/fileset/internal.nix

@@ -874,6 +874,8 @@ rec {
       throw "lib.fileset.${function}: This function is currently not supported in pure evaluation mode, since it currently relies on `builtins.fetchGit`. See https://github.com/NixOS/nix/issues/9292."
     else if ! isPath path then
       throw "lib.fileset.${function}: Expected the ${argument} to be a path, but it's a ${typeOf path} instead."
+    else if pathType path != "directory" then
+      throw "lib.fileset.${function}: Expected the ${argument} (${toString path}) to be a directory, but it's a file instead."
     # We can identify local working directories by checking for .git,
     # see https://git-scm.com/docs/gitrepository-layout#_description.
     # Note that `builtins.fetchGit` _does_ work for bare repositories (where there's no `.git`),

lib/fileset/tests.sh

@@ -1317,6 +1317,12 @@ rm -rf -- *
 expectFailure 'gitTracked null' 'lib.fileset.gitTracked: Expected the argument to be a path, but it'\''s a null instead.'
 expectFailure 'gitTrackedWith {} null' 'lib.fileset.gitTrackedWith: Expected the second argument to be a path, but it'\''s a null instead.'
 
+# The path must be a directory
+touch a
+expectFailure 'gitTracked ./a' 'lib.fileset.gitTracked: Expected the argument \('"$work"'/a\) to be a directory, but it'\''s a file instead'
+expectFailure 'gitTrackedWith {} ./a' 'lib.fileset.gitTrackedWith: Expected the second argument \('"$work"'/a\) to be a directory, but it'\''s a file instead'
+rm -rf -- *
+
 # The path has to contain a .git directory
 expectFailure 'gitTracked ./.' 'lib.fileset.gitTracked: Expected the argument \('"$work"'\) to point to a local working tree of a Git repository, but it'\''s not.'
 expectFailure 'gitTrackedWith {} ./.' 'lib.fileset.gitTrackedWith: Expected the second argument \('"$work"'\) to point to a local working tree of a Git repository, but it'\''s not.'