After I pushed a change set from a working copy to the Master repository, I ran Update on the Master to bring the files up to date. I have a hook in the update command to run a script to upload the changes to the server. It was at this point that the CPU usage went to %120-%150 and stayed there until I quit the application. (Note that the application did not hang; it was still somewhat responsive to commands and I was able to quit normally—no force quit.) Attached is a sample of the process while it was churning.
I'm running MacHg 0.9.14 d9af1d818c0c (I did not see this behavior when updating on the previous version)