prepare_tempfile_object(): new function, extracted from create_tempfile()

prepare_tempfile_object(): new function, extracted from create_tempfile()

This makes the next step easier.

The old code used to use "path" to set the initial length of
tempfile->filename. This was not helpful because path was usually
relative whereas the value stored to filename will be absolute. So
just initialize the length to 0.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>