client: use extend in fs.readlink
client: use extend in fs.readlink

This is followup for
https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1878207/4/client/utils/fs.py#380

Bug: 1010816
Change-Id: I2c4bea43fd9939f40ad60736087fe96096d1263c
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1880420
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py