When loading shallow grafts, we add each of the grafting commits to the
grafts backed by ".git/info/grafts". Keeping track of both grafts
separately, but partially storing them in a common grafts structure is
likely to lead to inconsistencies. In fact, there already are
inconsistencies if refreshing shallow grafts at a later point, as we
only refresh the shallows, but not the normal grafts in that case.

Disentangle both grafting stores and instead check both separately when
parsing commits.