fast-import: fix buffer overflow in dump_tags

When creating a new annotated tag, we sprintf the refname into a static-sized buffer. If we have an absurdly long tagname, like: git init repo && cd repo && git commit --allow-empty -m foo && git tag -m message mytag && git fast-export mytag | perl -lpe '/^tag/ and s/mytag/"a" x 8192/e' | git fast-import <input we'll overflow the buffer. We can fix it by using a strbuf. Signed-off-by: Jeff King <peff@peff.net> Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu> Reviewed-by: Ronnie Sahlberg <sahlberg@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>