Fix nodes._check_initialpaths_for_relpath for dirs

Ref: https://github.com/pytest-dev/pytest/issues/4321#issuecomment-436951894

Hardens some of the not many tests affected by this:

1. `testing/test_session.py::test_rootdir_option_arg` displayed:

> root/test_rootdir_option_arg2/test_rootdir_option_arg.py

2. `test_cmdline_python_namespace_package` displayed "hello/" prefix for:

> hello/test_hello.py::test_hello
> hello/test_hello.py::test_other

src/_pytest/nodes.py

@@ -447,7 +447,7 @@ class Collector(Node):
 def _check_initialpaths_for_relpath(session, fspath):
     for initial_path in session._initialpaths:
         if fspath.common(initial_path) == initial_path:
-            return fspath.relto(initial_path.dirname)
+            return fspath.relto(initial_path)
 
 
 class FSCollector(Collector):

testing/acceptance_test.py

@@ -663,11 +663,11 @@ class TestInvocationVariants(object):
         assert result.ret == 0
         result.stdout.fnmatch_lines(
             [
-                "*test_hello.py::test_hello*PASSED*",
-                "*test_hello.py::test_other*PASSED*",
-                "*test_world.py::test_world*PASSED*",
-                "*test_world.py::test_other*PASSED*",
-                "*4 passed*",
+                "test_hello.py::test_hello*PASSED*",
+                "test_hello.py::test_other*PASSED*",
+                "ns_pkg/world/test_world.py::test_world*PASSED*",
+                "ns_pkg/world/test_world.py::test_other*PASSED*",
+                "*4 passed in*",
             ]
         )
 

testing/test_nodes.py

@@ -1,3 +1,5 @@
+import py
+
 import pytest
 from _pytest import nodes
 
@@ -29,3 +31,23 @@ def test_std_warn_not_pytestwarning(testdir):
     )
     with pytest.raises(ValueError, match=".*instance of PytestWarning.*"):
         items[0].warn(UserWarning("some warning"))
+
+
+def test__check_initialpaths_for_relpath():
+    """Ensure that it handles dirs, and does not always use dirname."""
+    cwd = py.path.local()
+
+    class FakeSession:
+        _initialpaths = [cwd]
+
+    assert nodes._check_initialpaths_for_relpath(FakeSession, cwd) == ""
+
+    sub = cwd.join("file")
+
+    class FakeSession:
+        _initialpaths = [cwd]
+
+    assert nodes._check_initialpaths_for_relpath(FakeSession, sub) == "file"
+
+    outside = py.path.local("/outside")
+    assert nodes._check_initialpaths_for_relpath(FakeSession, outside) is None

testing/test_session.py

@@ -323,7 +323,11 @@ def test_rootdir_option_arg(testdir, monkeypatch, path):
 
     result = testdir.runpytest("--rootdir={}".format(path))
     result.stdout.fnmatch_lines(
-        ["*rootdir: {}/root, inifile:*".format(testdir.tmpdir), "*1 passed*"]
+        [
+            "*rootdir: {}/root, inifile:*".format(testdir.tmpdir),
+            "root/test_rootdir_option_arg.py *",
+            "*1 passed*",
+        ]
     )