[FIX] crm: merge leads in multi-company

When merging leads with a company set different
than the current user company,
it failed because the notification email (the message
written in the thread summarizing the merge) couldn't
read the company name (the `name_get` call on the company)
because of the multi-company record rule.

`name_get` can be safely called as `sudo`, as it does
nothing else than returning the name of the record. This is
already done for the many2one links in the web client anyway
(You can see leads from a company different than yours,
you can see the company name to which the leads belongs to,
but you can't click on the company many2one link)

opw-677986