Simplify strbuf uses in archive-tar.c using strbuf API

This is just cleaner way to deal with strbufs, using its API rather than
reinventing it in the module (e.g. strbuf_append_string is just the plain
strbuf_addstr function, and it was used to perform what strbuf_addch does
anyways).

Signed-off-by: Junio C Hamano <gitster@pobox.com>