You can almost get away with just calling "strip_suffix_mem"
on a strbuf's buf and len fields. But we also need to move
the NUL-terminator to satisfy strbuf's invariants. Let's
provide a convenience wrapper that handles this.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>