fast-export: report SHA-1 instead of gibberish when marks exist already Cc: Pieter de Bie <pdebie@ai.rug.nl> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Max Horn <max@quendi.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>