Merge branch 'nd/index-pack-one-fd-per-thread' into maint

We used to disable threaded "git index-pack" on platforms without
thread-safe pread(); use a different workaround for such
platforms to allow threaded "git index-pack".

* nd/index-pack-one-fd-per-thread:
index-pack: work around thread-unsafe pread()

diff --cc Makefile
Simple merge

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

diff --cc config.mak.uname
Simple merge