cgroups: Handle nil repo (pick into 15-0-stable)
This picks 00e44ba6 into the stable branch. The original MR is here !4572 (merged)
If a command's repo is nil, handle it gracefully by adding the command to a cgroup by using the command arguments as a key for the circular hash.
While we're at it, fix a test that was doing the checksum incorrectly. This didn't make a difference in the test however, because we were using the groupID we calculated ourselves to check for the location of the cgroup file.
Changelog: fixed
Edited by John Cai