Fix an issue where sparse checkout wasn't configured correctly

This was affecting the performance of `Gitlab::Git::Repository#squash`.

lib/gitlab/git/repository.rb

@@ -1519,7 +1519,7 @@ module Gitlab
         if sparse_checkout_files
           # Create worktree without checking out
           run_git!(base_args + ['--no-checkout', worktree_path], env: env)
-          worktree_git_path = run_git!(%w(rev-parse --git-dir), chdir: worktree_path)
+          worktree_git_path = run_git!(%w(rev-parse --git-dir), chdir: worktree_path).chomp
           configure_sparse_checkout(worktree_git_path, sparse_checkout_files)

spec/lib/gitlab/git/repository_spec.rb

@@ -20,6 +20,7 @@ describe Gitlab::Git::Repository, seed_helper: true do
   let(:repository) { Gitlab::Git::Repository.new('default', TEST_REPO_PATH, '') }
   let(:storage_path) { TestEnv.repos_path }
+  let(:user) { build(:user) }
   describe '.create_hooks' do
     let(:repo_path) { File.join(storage_path, 'hook-test.git') }
@@ -693,7 +694,6 @@ describe Gitlab::Git::Repository, seed_helper: true do
   describe '#remote_tags' do
     let(:remote_name) { 'upstream' }
     let(:target_commit_id) { SeedRepo::Commit::ID }
-    let(:user) { create(:user) }
     let(:tag_name) { 'v0.0.1' }
     let(:tag_message) { 'My tag' }
     let(:remote_repository) do
@@ -1711,7 +1711,6 @@ describe Gitlab::Git::Repository, seed_helper: true do
     shared_examples "user deleting a branch" do
       let(:project) { create(:project, :repository) }
       let(:repository) { project.repository.raw }
-      let(:user) { create(:user) }
       let(:branch_name) { "to-be-deleted-soon" }
       before do
@@ -1795,7 +1794,6 @@ describe Gitlab::Git::Repository, seed_helper: true do
       Gitlab::Git::Repository.new('default', TEST_MUTABLE_REPO_PATH, '')
     end
     let(:source_sha) { '913c66a37b4a45b9769037c55c2d238bd0942d2e' }
-    let(:user) { build(:user) }
     let(:target_branch) { 'test-merge-target-branch' }
     before do
@@ -1848,7 +1846,6 @@ describe Gitlab::Git::Repository, seed_helper: true do
     end
     let(:branch_head) { '6d394385cf567f80a8fd85055db1ab4c5295806f' }
     let(:source_sha) { 'cfe32cf61b73a0d5e9f13e774abde7ff789b1660' }
-    let(:user) { build(:user) }
     let(:target_branch) { 'test-ff-target-branch' }
     before do
@@ -2167,6 +2164,47 @@ describe Gitlab::Git::Repository, seed_helper: true do
         expect { subject }.to raise_error(Gitlab::Git::CommandError, 'error')
       end
     end
+
+    describe '#squash' do
+      let(:squash_id) { '1' }
+      let(:branch_name) { 'fix' }
+      let(:start_sha) { '4b4918a572fa86f9771e5ba40fbd48e1eb03e2c6' }
+      let(:end_sha) { '12d65c8dd2b2676fa3ac47d955accc085a37a9c1' }
+
+      subject do
+        opts = {
+          branch: branch_name,
+          start_sha: start_sha,
+          end_sha: end_sha,
+          author: user,
+          message: 'Squash commit message'
+        }
+
+        repository.squash(user, squash_id, opts)
+      end
+
+      context 'sparse checkout' do
+        let(:expected_files) { %w(files files/js files/js/application.js) }
+
+        before do
+          allow(repository).to receive(:with_worktree).and_wrap_original do |m, *args|
+            m.call(*args) do
+              worktree_path = args[0]
+              files_pattern = File.join(worktree_path, '**', '*')
+              expected = expected_files.map do |path|
+                File.expand_path(path, worktree_path)
+              end
+
+              expect(Dir[files_pattern]).to eq(expected)
+            end
+          end
+        end
+
+        it 'checkouts only the files in the diff' do
+          subject
+        end
+      end
+    end
   end
   def create_remote_branch(repository, remote_name, branch_name, source_branch_name)