After the previous fix introducing a deepcopy, the pregenerate of the
assets became slower because of the many call to get_manifest (in loop,
recursively)
Since the manifest is immutable here and won't go outside of the call,
we can use the lower level version.
closesodoo/odoo#146602
X-original-commit: acb671f4d1b9c53c9e7d91414e847f26e8026ca3
Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>