Merge branch 'jk/virtual-objects-do-exist'

A recent update broke "is this object available to us?" check for
well-known objects like an empty tree (which should yield "yes",
even when there is no on-disk object for an empty tree), which has
been corrected.

* jk/virtual-objects-do-exist:
rev-list: allow cached objects in existence check

diff --cc builtin/rev-list.c
Simple merge

diff --cc t/t1060-object-corruption.sh
Simple merge