Example Org file:

* Parent :foo:bar:baz:
** Child :foo:bar:spam:

Invoking org-set-tags-command with point on Child prepopulates the
minibuffer prompt with only the tags :spam:

This is because org-get-tags doesn't distinguish between inherited only
tags and inherited tags which are also explicitly set on a heading, so
org-set-tags-command treats :foo:bar: as inherited only rather than also
set locally on the heading.

This is undesirable because having tags set directly on a heading has
different semantics even if they are also inherited (e.g., the TAGS
special property, or when headings will be refiled to a different
location later).

Attached patch.