bot_update: Use 'gclient' from same repository.

Currently, 'bot_update' uses the 'gclient' that is on the system path.
Now, it will use the 'gclient.py' that is in the same 'depot_tools'
checkout as the 'bot_update' recipe module.

Also don't ignore "git_cache" move errors.

BUG=664254,663990,663440
TEST=None

Review-Url: https://codereview.chromium.org/2492963002

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()