Fix bug with multiple archiving to sibling.

Fix bug with multiple archiving to sibling.

Calling `org-archive-to-archive-sibling' caused problems when used
several times in a row.  The new killed subtree would be appended to
the current head of the kill-ring, instead of being pushed there.  The
reason for this is that the command `kill-region', which is called by
`org-cut-subtree' changes the variable `this-command', so that during
the next archive command, `kill-region' will think that the previous
command was also a kill and do the append.

Normally this is wanted, to make sure that several kill commands
create one big yank string.  But in this case it has to be avoided.
The patch fixes the problem by let-binding (and therefore protecting)
the value of `this-command' around the call to `org-cut-subtree'.

Reported by Bernt Hansen.