sequencer: improve error message when an OID could not be parsed

The interactive rebase simply complains about an "invalid line" when the
object hash of, say, a `pick` line could not be parsed.

Let's tell the user what happened in a little more detail.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -2137,7 +2137,8 @@ static int parse_insn_line(struct repository *r, struct todo_item *item,
 	item->arg_len = (int)(eol - item->arg);
 
 	if (status < 0)
-		return -1;
+		return error(_("could not parse '%.*s'"),
+			     (int)(end_of_object_name - bol), bol);
 
 	item->commit = lookup_commit_reference(r, &commit_oid);
 	return !item->commit;