Merge branch 'eb/no-pthreads'

Allow us build with NO_PTHREADS=NoThanks compilation option.

* eb/no-pthreads:
  Handle atexit list internaly for unthreaded builds
  pack-objects: set number of threads before checking and warning
  index-pack: fix compilation with NO_PTHREADS

diff --cc builtin/clone.c
Simple merge

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

diff --cc git-compat-util.h
Simple merge

diff --cc run-command.c
Simple merge

diff --cc shallow.c
Simple merge