In this code, we try to convert both "foo.idx" and "foo"
into "foo.pack". By stripping the suffix, we can avoid a
confusing use of strbuf_splice, and make it clear that both
cases are adding ".pack" to the end.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>