gitserver deletes all repositories if the disk size is ~= SRC_REPOS_DESIRED_FREE_GB

Created by: ggilmore

https://github.com/sourcegraph/sourcegraph/blob/1ed7c5fbd6300aff6436478d1ebd91d02764c8e7/cmd/gitserver/main.go#L27

Gitserver periodically runs a cleanup job so that it always keeps a certain amount of disk space free (default 10GB). However, if the actual disk size is ~= SRC_REPOS_DESIRED_FREE_GB, then gitsever will delete all of the repos on the instance as part of the cleanup process. This forces gitserver to reclone all those repos again, which results in bad/broken UX.

One way of fixing this is to make the cleanup threshold percentage-based instead of a fixed number.

cc @sourcegraph/core-services