Shallow clone doesn't work with concurrent builds

!188 (merged) introduced the experimental GIT_DEPTH variable for gitlab-ci. I just added that to our build, and there seems to be a race condition.

gitlab-ci-multi-runner 1.3.2 (0323456)
Using Docker executor with image eu.gcr.io/ring-web-ci/ring-web-ci:latest ...
Pulling docker image eu.gcr.io/ring-web-ci/ring-web-ci:latest ...
Running on runner-f775b9ab-project-127-concurrent-0 via runner-f775b9ab-1468139271-894abd4b...
Fetching changes for task/simplify-parameters with git depth set to 50...
fatal: Unable to create '/builds/classmarkets/ring-web/.git/shallow.lock': File exists.

If no other git process is currently running, this probably means a
git process crashed in this repository earlier. Make sure no other git
process is running and remove the file manually to continue.

ERROR: Build failed: exit code 1