Merge branch 'nd/index-pack-threaded-fixes'

"index-pack --verify-stat" used a few counters outside protection
of mutex, possibly showing incorrect numbers.

* nd/index-pack-threaded-fixes:
index-pack: guard nr_resolved_deltas reads by lock
index-pack: protect deepest_delta in multithread code

diff --cc builtin/index-pack.c
Simple merge