Move sha1_file_to_archive into libgit

Move sha1_file_to_archive into libgit

When the specfile (export-subst) attribute was introduced, it added a
dependency from archive-{tar|zip}.c to builtin-archive.c. This broke the
support for archive-operations in libgit.a since builtin-archive.o doesn't
belong in libgit.a.

This patch moves the functions required by libgit.a from builtin-archive.c
to the new file archive.c (which becomes part of libgit.a).

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>