call magit-OPNAME-action-hook, fixes #802

Since 46467190 `magit-section-action' was looking for a section of
type (run-hook-with-args-until-success magit-*opname*-action-hook)
instead of actually calling the hook.
make :eval kludge added in last commit unnecessary

Instead of adding `(:eval ...)' as another special kind of
`magit-section-case' CLAUSE, run the `magit-section-action'
hook inside the existing `t' special clause.