[FIX] mail: avoid a stack overflow during loading

When the amount of mail messages is big enough (about 1500?), the
recursion goes deeper and deeper, until the Javascript limit raises an
infinite recursion error.

This commit fixes the issue by avoiding to run an inner
_flushUpdateCycle with the update if the change is not needed, so the
second time that it is reached it does not start a new update, which
would have run another flush update cycle.

closes odoo/odoo#62076

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>