dgit: clone multisuite works even without --no-rm-on-error.

dgit: clone multisuite works even without --no-rm-on-error.

This is slightly subtle.

We need $rmonerror in cmd_clone to be cleared.  cmd_clone does that
only when clone itself returns.  The multisuite plumbing means that
currently, clone returns only in the child which set up the tree and
set up the first suite.  Unsetting $rmonerror there is correct because
we want to keep the tree on success, and the parent will remove it if
there is a later failure.  (Ie, the child remains responsible for
removing the tree if it itself fails.)

In the parent, we get $multi_fetched==1 if not only our original clone
child succeeded, but all the other fetches worked too.  We are now
responsible for the tree.  If our final tasks are successful, again,
we need not to delete the tree.

Closes:#867434.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>