Does not handle index file corrupt

The Git index file got corrupted causing the build to fail and the runner is not able to recover.

It should be able to handle this by deleting the index file and resetting the repo (https://stackoverflow.com/a/1115956)

Running with gitlab-runner 10.3.0 (5cf5e19a)
  on pc (7f360386)
Using Shell executor...
Running on pc...
Fetching changes...
error: bad signature
fatal: index file corrupt
error: bad signature
fatal: index file corrupt
Checking out b94a0961 as branch-1...
error: bad signature
fatal: index file corrupt
Uploading artifacts...
Uploading artifacts to coordinator... ok            id=116733 responseStatus=201 Created token=4XENpT_s
ERROR: Job failed: exit status 128