interpret-trailers: refactor create_in_place_tempfile()

interpret-trailers: refactor create_in_place_tempfile()

Refactor create_in_place_tempfile() in preparation for moving it
to tralier.c. Change the return type to return a `struct tempfile*`
instead of a `FILE*` so that we can remove the file scope tempfile
variable. Since 076aa2cbda5 (tempfile: auto-allocate tempfiles on
heap, 2017-09-05) it has not been necessary to make tempfile varibales
static so this is safe. Also use error() and return NULL in place of
die() so the caller can exit gracefully and use find_last_dir_sep()
rather than strchr() to find the parent directory.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>