gitserver: use an atomic alternative to os.RemoveAll (!3985) · Merge requests · Administrator / sourcegraph · GitLab

Administrator requested to merge 3972-atomic-cleanup into master May 13, 2019

Created by: ijt

Fixes https://github.com/sourcegraph/sourcegraph/issues/3972

Test plan: This PR includes a unit test, including a sub-test for atomicity.