Merge branch 'nd/pthreads'

The codebase has been cleaned up to reduce "#ifndef NO_PTHREADS".

* nd/pthreads:
Clean up pthread_create() error handling
read-cache.c: initialize copy_len to shut up gcc 8
read-cache.c: reduce branching based on HAVE_THREADS
read-cache.c: remove #ifdef NO_PTHREADS
pack-objects: remove #ifdef NO_PTHREADS
preload-index.c: remove #ifdef NO_PTHREADS
grep: clean up num_threads handling
grep: remove #ifdef NO_PTHREADS
attr.c: remove #ifdef NO_PTHREADS
name-hash.c: remove #ifdef NO_PTHREADS
index-pack: remove #ifdef NO_PTHREADS
send-pack.c: move async's #ifdef NO_PTHREADS back to run-command.c
run-command.h: include thread-utils.h instead of pthread.h
thread-utils: macros to unconditionally compile pthreads API

diff --cc Makefile
Simple merge

diff --cc builtin/grep.c
Simple merge

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

diff --cc pack-objects.h
Simple merge

diff --cc preload-index.c
Simple merge

diff --cc read-cache.c
Simple merge

diff --cc run-command.c
Simple merge