unpack-trees: reuse (still valid) cache-tree from src_index

unpack-trees: reuse (still valid) cache-tree from src_index

We do n-way merge by walking the source index and n trees at the same
time and add merge results to a new temporary index called o->result.
The merge result for any given path could be either

- keep_entry(): same old index entry in o->src_index is reused
- merged_entry(): either a new entry is added, or an existing one updated
- deleted_entry(): one entry from o->src_index is removed

For some reason [1] we keep making sure that the source index's
cache-tree is still valid if used by o->result: for all those
merged/deleted entries, we invalidate the same path in o->src_index,
so only cache-trees covering the "keep_entry" parts remain good.

Because of this, the cache-tree from o->src_index can be perfectly
reused in o->result. And in fact we already rely on this logic to
reuse untracked cache in edf3b90553 (unpack-trees: preserve index
extensions - 2017-05-08). Move the cache-tree to o->result before
doing cache_tree_update() to reduce hashing cost.

Since cache_tree_update() has risen up as one of the most expensive
parts in unpack_trees() after the last few patches. This does help
reduce unpack_trees() time significantly (on webkit.git):

    before       after
  --------------------------------------------------------------------
    0.080394752  0.051258167 s:  read cache .git/index
    0.216010838  0.212106298 s:  preload index
    0.008534301  0.280521764 s:  refresh index
    0.251992198  0.218160442 s:   traverse_trees
    0.377031383  0.374948191 s:   check_updates
    0.372768105  0.037040114 s:   cache_tree_update
    1.045887251  0.672031609 s:  unpack_trees
    0.314983512  0.317456290 s:  write index, changed mask = 2e
    0.062572653  0.038382654 s:    traverse_trees
    0.000022544  0.000042731 s:    check_updates
    0.073795585  0.050930053 s:   unpack_trees
    0.073807557  0.051099735 s:  diff-index
    1.938191592  1.614241153 s: git command: git checkout -

[1] I'm pretty sure the reason is an oversight in 34110cd4e3 (Make
    'unpack_trees()' have a separate source and destination index -
    2008-03-06). That patch aims to _not_ update the source index at
    all. The invalidation should have been done on o->result in that
    patch. But then there was no cache-tree on o->result even then so
    it's pointless to do so.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>