git_async_cat: unref pipes on EOF from git->cleanup
We avoided a managed circular reference in
10ee3548084c125f
but introduced a pipe FD leak, instead. So handle the EOF
we get when the "git cat-file --batch" process exits and
closes its stdout FD.
v2: remove ->close entirely. PublicInbox::Git->cleanup
handles all cleanup. This prevents us from inadvertantly
deleting the {async_cat} field associated with a different
pipe than the one GAC is monitoring.
Fixes: 10ee3548084c125f ("git_async_cat: remove circular reference")