[FIX] mail: prevent record discard on new attachment
[FIX] mail: prevent record discard on new attachment

Open a record having a chatter, edit a field, add an attachment, leave
the page. The discard warning popup in not shown.

When an attachment is added, it reloads the record to fetch the new
`attachment_message_count`. Reloading the view was discarding the
changes because the `keepChanges` option was not set and defaults to
`false`. The changes were discarded thus the internal `_isDirty` flag
evaluated to `false` thus the popup was not shown as nothing would have
been saved.

The option was set in another `attachment_message_count` reload already,
in `chatter.js@_openComposer`.

opw-1974147

closes odoo/odoo#33180

Signed-off-by: Alexandre Kühn (aku) <22220c18caede9806cb42c8e2b41136d711673bf@odoo.com>
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo