Fix the format of rugged alternate directory list

Fixes #39046

lib/gitlab/git/repository.rb

@@ -1230,6 +1230,7 @@ module Gitlab
           relative_paths.map { |d| File.join(path, d) }
         else
           Gitlab::Git::Env.all.values_at(*ALLOWED_OBJECT_DIRECTORIES_VARIABLES)
+            .flatten
             .compact
             .flat_map { |d| d.split(File::PATH_SEPARATOR) }
         end

spec/lib/gitlab/git/repository_spec.rb

@@ -87,7 +87,7 @@ describe Gitlab::Git::Repository, seed_helper: true do
             'GIT_OBJECT_DIRECTORY_RELATIVE' => './objects/foo',
             'GIT_ALTERNATE_OBJECT_DIRECTORIES_RELATIVE' => ['./objects/bar', './objects/baz'],
             'GIT_OBJECT_DIRECTORY' => 'ignored',
-            'GIT_ALTERNATE_OBJECT_DIRECTORIES' => 'ignored:ignored',
+            'GIT_ALTERNATE_OBJECT_DIRECTORIES' => %w[ignored ignored],
             'GIT_OTHER' => 'another_env'
           })
         end
@@ -104,7 +104,7 @@ describe Gitlab::Git::Repository, seed_helper: true do
         before do
           allow(Gitlab::Git::Env).to receive(:all).and_return({
             'GIT_OBJECT_DIRECTORY' => 'foo',
-            'GIT_ALTERNATE_OBJECT_DIRECTORIES' => 'bar:baz',
+            'GIT_ALTERNATE_OBJECT_DIRECTORIES' => %w[bar baz],
             'GIT_OTHER' => 'another_env'
           })
         end