archive: unify file attribute handling

archive: unify file attribute handling

Now that all file attribute handling for git archive has moved to archive.c,
we can unexport sha1_file_to_archive() and is_archive_path_ignored() even
disappears.

Add setup_archive_check(), modelled after similar functions used in the code
of other commands that support multiple file attributes.

Also remove convert_to_archive(), as it's only remaining function with
attribute handling gone was to call format_subst() if commit was not NULL,
which is now checked in sha1_file_to_archive().

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>