diff-index: keep the original index intact

diff-index: keep the original index intact

When comparing the index and a tree, we used to read the contents of the
tree into stage #1 of the index and compared them with stage #0.  In order
not to lose sight of entries originally unmerged in the index, we hoisted
them to stage #3 before reading the tree.

Commit d1f2d7e (Make run_diff_index() use unpack_trees(), not read_tree(),
2008-01-19) changed all this.  These days, we instead use unpack_trees()
API to traverse the tree and compare the contents with the index, without
modifying the index at all.  There is no reason to hoist the unmerged
entries to stage #3 anymore.

Signed-off-by: Junio C Hamano <gitster@pobox.com>