log_tree_diff: die when we fail to parse a commit

We currently call parse_commit and then assume we can
dereference the resulting "tree" struct field. If parsing
failed, however, that field is NULL and we end up
segfaulting.

Instead of a segfault, let's print an error message and die
a little more gracefully.

Note that this should never happen in practice, but may
happen in a corrupt repository.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -341,6 +341,13 @@ int parse_commit(struct commit *item)
 	return ret;
 }
 
+void parse_commit_or_die(struct commit *item)
+{
+	if (parse_commit(item))
+		die("unable to parse commit %s",
+		    item ? sha1_to_hex(item->object.sha1) : "(null)");
+}
+
 int find_commit_subject(const char *commit_buffer, const char **subject)
 {
 	const char *eol;