[FIX] disgusting hack to skip around race condition

[FIX] disgusting hack to skip around race condition

When clicking on an action button, form view first saves then executes
the action. In doing so, it causes the o2m field to call
ListView#reload_content twice without waiting for the reloading to
end, generating a race condition in ListView.List#render_dataset.

As a result, the read_slice callback fills the list twice (once for
each #reload_content call), resulting in everything being duplicated.

Clear the content of the records collection before adding stuff.

note: collections should dedup on ids.

lp bug: https://launchpad.net/bugs/877965 fixed

bzr revid: xmo@openerp.com-20111025085112-q38q8m0k2mopwadg