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>

builtin-fast-export.c

diff --git a/builtin-fast-export.c b/builtin-fast-export.c
index 45786ef..1bf4e06 100644
--- a/builtin-fast-export.c
+++ b/builtin-fast-export.c
@@ -419,7 +419,7 @@
 			die ("Could not read blob %s", sha1_to_hex(sha1));
 
 		if (object->flags & SHOWN)
-			error("Object %s already has a mark", sha1);
+			error("Object %s already has a mark", sha1_to_hex(sha1));
 
 		mark_object(object, mark);
 		if (last_idnum < mark)