Use git_mkstemp_mode instead of plain mkstemp to create object files

Use git_mkstemp_mode instead of plain mkstemp to create object files

We used to unnecessarily give the read permission to group and others,
regardless of the umask, which isn't serious because the objects are
still protected by their containing directory, but isn't necessary
either.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>