bot_update: Use 'gclient' from same repository.

git_cache.py

Index: git_cache.py
diff --git a/git_cache.py b/git_cache.py
index 60dfa1c1fa637b26b2e24556052dfaefac0294c6..0f66de72e9746b170df01982ccbf6ebaf0ac1936 100755
--- a/git_cache.py
+++ b/git_cache.py
@@ -439,16 +439,9 @@ class Mirror(object):
       self._fetch(tempdir or self.mirror_path, verbose, depth)
     finally:
       if tempdir:
-        try:
-          if os.path.exists(self.mirror_path):
-            gclient_utils.rmtree(self.mirror_path)
-          os.rename(tempdir, self.mirror_path)
-        except OSError as e:
-          # This is somehow racy on Windows.
-          # Catching OSError because WindowsError isn't portable and
-          # pylint complains.
-          self.print('Error moving %s to %s: %s' % (tempdir, self.mirror_path,
-                                                    str(e)))
+        if os.path.exists(self.mirror_path):
+          gclient_utils.rmtree(self.mirror_path)
+        os.rename(tempdir, self.mirror_path)
       if not ignore_lock:
         lockfile.unlock()