Merge branch 'jh/loose-object-dirs-creation-race' into maint

Two processes creating loose objects at the same time could have
failed unnecessarily when the name of their new objects started
with the same byte value, due to a race condition.

* jh/loose-object-dirs-creation-race:
  sha1_file.c:create_tmpfile(): Fix race when creating loose object dirs

diff --cc sha1_file.c
Simple merge