Reversion introduced in version 8.20201007, one release after the 1st
release with the extension.
Surprisingly, hClose can hang if another thread is reading from the
handle. This is because it uses takeMVar.
The use of cancel here does mean that, if receiveMessageAddonProcess
or Remote.External.AsyncExtension.receiveloop allocated some resource in
a non-async-exception safe way, they might not get a chance to clean it up.
They do not appear to, and anyway, this only happens when git-annex is
shutting down, so any recource that did leak would not be a problem.
This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.
7776677a5f