If a subdir exists and is empty, go ahead and clone into it.

This will make it easier to support git-submodule and gclient at
the same time.

Review URL: http://codereview.chromium.org/9692022

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@126428 0039d316-1c4b-4281-b951-d872f2087c98

gclient_scm.py

@@ -219,7 +219,9 @@ class GitWrapper(SCMWrapper):
       # hash is also a tag, only make a distinction at checkout
       rev_type = "hash"
 
-    if not os.path.exists(self.checkout_path):
+    if not os.path.exists(self.checkout_path) or (
+        os.path.isdir(self.checkout_path) and
+        not os.listdir(self.checkout_path)):
       gclient_utils.safe_makedirs(os.path.dirname(self.checkout_path))
       self._Clone(revision, url, options)
       files = self._Capture(['ls-files']).splitlines()