Fix race condition with git_pillar mountpoints
Fix race condition with git_pillar mountpoints

When multiple independent jobs are attempting to compile git_pillar at
the same time, and they are using mountpoints, this results in the repo
linkdir being removed during the compile process, causing some jobs to
fail. This commit implements locking to avoid this.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt