Git checkout crashes the server

[marijnh]

When I have distfs running on ProseMirror and switch to a different branch, the process will crash, and block all further access to the dist directory. This happens even if there are no git-controlled files in dist.

One theory is that the rapid touching of a ton of files in the source directory causes our scary delete-file-to-make-inotify-work hack to somehow lock up the server process. Need to do more strace-ing to debug.