Fetching repositories twice does not work

[lberki]

./compile.sh
cat > base_workspace/WORKSPACE <<EOF
git_repository(
    name = "protobuf",
    remote = "https://github.com/google/protobuf",
    commit = "HEAD"
)
EOF
cd base_workspace
../output/bazel --batch build @protobuf//:protoc
../output/bazel --batch build @protobuf//:protoc

The second execution results in an ugly exception:

java.lang.RuntimeException: Unrecoverable error while evaluating node 'GIT_CLONE:https://github.com/google/protobuf -> /usr/local/google/home/lberki/.cache/bazel/_bazel_lberki/b054b3aab4025af98dc2601e4ff14214/external/protobuf (HEAD) submodules: false' (requested by nodes 'GIT_REPOSITORY:@protobuf')
    at com.google.devtools.build.skyframe.ParallelEvaluator$Evaluate.run(ParallelEvaluator.java:745)
    at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$1.run(AbstractQueueVisitor.java:346)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
Caused by: org.eclipse.jgit.api.errors.JGitInternalException: Destination path "protobuf" already exists and is not an empty directory
    at org.eclipse.jgit.api.CloneCommand.init(CloneCommand.java:152)
    at org.eclipse.jgit.api.CloneCommand.call(CloneCommand.java:132)
    at com.google.devtools.build.lib.bazel.repository.GitCloneFunction.compute(GitCloneFunction.java:68)
    at com.google.devtools.build.skyframe.ParallelEvaluator$Evaluate.run(ParallelEvaluator.java:709)
    ... 4 more

[lberki]

Accidentally submitted twice. Duplicate is #330.