Consistently return `false` from `ED_object_parent_set()` when parenting
is not possible. Before, when parent and child were the same object, the
function would return `true` even though the parent-child relation was
not made.
Just returning `false` in the `parent == child` case would break the
parenting operator, as `false` stops its loop over all selected objects.
This tight coupling caused T82312. The loop now has its own check for
this, so that it properly continues, and the implementation of
`ED_object_parent_set()` is decoupled from its surrounding code.
No functional changes.
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