Fix tests when running in git linked worktree.

tests/conftest.py

@@ -169,10 +169,16 @@ def isolate(tmpdir):
 @pytest.fixture(scope='session')
 def pip_src(tmpdir_factory):
     def not_code_files_and_folders(path, names):
-        # In the root directory, ignore all folders except "src"
+        # In the root directory...
         if path == SRC_DIR:
+            # ignore all folders except "src"
             folders = {name for name in names if os.path.isdir(path / name)}
-            return folders - {"src"}
+            to_ignore = folders - {"src"}
+            # and ignore ".git" if present (which may be a file if in a linked
+            # worktree).
+            if ".git" in names:
+                to_ignore.add(".git")
+            return to_ignore
 
         # Ignore all compiled files and egg-info.
         ignored = list()

tests/unit/test_vcs.py

@@ -123,15 +123,19 @@ def test_should_add_vcs_url_prefix(vcs_cls, remote_url, expected):
     assert actual == expected
 
 
-@patch('pip._internal.vcs.git.Git.get_revision')
 @patch('pip._internal.vcs.git.Git.get_remote_url')
+@patch('pip._internal.vcs.git.Git.get_revision')
+@patch('pip._internal.vcs.git.Git.get_subdirectory')
 @pytest.mark.network
-def test_git_get_src_requirements(mock_get_remote_url, mock_get_revision):
+def test_git_get_src_requirements(
+    mock_get_subdirectory, mock_get_revision, mock_get_remote_url
+):
     git_url = 'https://github.com/pypa/pip-test-package'
     sha = '5547fa909e83df8bd743d3978d6667497983a4b7'
 
     mock_get_remote_url.return_value = git_url
     mock_get_revision.return_value = sha
+    mock_get_subdirectory.return_value = None
 
     ret = Git.get_src_requirement('.', 'pip-test-package')