Fix ignoring git branch in Dockerfile.devel and Dockerfile.devel-gpu

[gallir]

The build argument TF_SERVING_VERSION_GIT_BRANCH was ignored in both dockerfiles, although it was correctly handled in .devel-mkl.

I just copied&pasted the same "git clone" lines from mkl. Now the command

docker build --pull -t $USER/tensorflow-serving-devel --build-arg TF_SERVING_VERSION_GIT_BRANCH="r2.18" -f tensorflow_serving/tools/docker/Dockerfile.devel .

uses the right branch.

Note: I realised it because there's a bug in tensorflow_serving/serving.bzl introduced two months ago by https://github.com/tensorflow/serving/commit/2c9e66b26e579258945bc3efe186a83e092485fd. I wanted to compile with r2.18 but the ARG in docker build didn't work and always used master.

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