fsck: detect submodule urls starting with dash

Urls with leading dashes can cause mischief on older
versions of Git. We should detect them so that they can be
rejected by receive.fsckObjects, preventing modern versions
of git from being a vector by which attacks can spread.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
static int fsck_commit(struct commit *commit, const char *data,
	unsigned long size, struct fsck_options *options)
{
	const char *buffer = data ?  data : get_commit_buffer(commit, &size);
	int ret = fsck_commit_buffer(commit, buffer, size, options);
	if (!data)
		unuse_commit_buffer(commit, buffer);
	return ret;
}

static int fsck_commit(struct commit *commit, const char *data,
	unsigned long size, struct fsck_options *options)
{
	const char *buffer = data ?  data : get_commit_buffer(commit, &size);
	int ret = fsck_commit_buffer(commit, buffer, size, options);
	if (!data)
		unuse_commit_buffer(commit, buffer);
	return ret;
}
