unpack-trees: add performance tracing
We're going to optimize unpack_trees() a bit in the following
patches. Let's add some tracing to measure how long it takes before
and after. This is the baseline ("git checkout -" on webkit.git, 275k
files on worktree)
performance: 0.
056651714 s: read cache .git/index
performance: 0.
183101080 s: preload index
performance: 0.
008584433 s: refresh index
performance: 0.
633767589 s: traverse_trees
performance: 0.
340265448 s: check_updates
performance: 0.
381884638 s: cache_tree_update
performance: 1.
401562947 s: unpack_trees
performance: 0.
338687914 s: write index, changed mask = 2e
performance: 0.
411927922 s: traverse_trees
performance: 0.
000023335 s: check_updates
performance: 0.
423697246 s: unpack_trees
performance: 0.
423708360 s: diff-index
performance: 2.
559524127 s: git command: git checkout -
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>