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

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

When two processes created one loose object file each, which fell
into the same fan-out bucket that previously did not have any
objects, they both tried to do an equivalent of

    mkdir .git/objects/$fanout &&
    chmod $shared_perm .git/objects/$fanout

before writing into their file .git/objects/$fanout/$remainder,
one of which could have failed unnecessarily when the second
invocation of mkdir found that the directory already has been
created by the first one.

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