Use config_invocation_dir for startdirs

`Session.startdir` and `TerminalReporter.startdir` appear to be
redundant given `Config.invocation_dir`.
Keep them for backward compatibility reasons, but use
`config.invocation_dir` for them.

src/_pytest/main.py

@@ -434,7 +434,7 @@ class Session(nodes.FSCollector):
         self.shouldfail = False
         self.trace = config.trace.root.get("collection")
         self._norecursepatterns = config.getini("norecursedirs")
-        self.startdir = py.path.local()
+        self.startdir = config.invocation_dir
         self._initialpaths = frozenset()
         # Keep track of any collected nodes in here, so we don't duplicate fixtures
         self._node_cache = {}

src/_pytest/terminal.py

@@ -234,7 +234,7 @@ class TerminalReporter(object):
         self._showfspath = None
 
         self.stats = {}
-        self.startdir = py.path.local()
+        self.startdir = config.invocation_dir
         if file is None:
             file = sys.stdout
         self._tw = _pytest.config.create_terminal_writer(config, file)