New NULL dereference in sequence.c

Hi all,

The last defect Sentry picked up has been removed, yay! At the same time, a new NULL dereference bug appeared, in sequencer.c:

    static int is_index_unchanged(void)
    {
        unsigned char head_sha1[20];
        struct commit *head_commit;

        if (!resolve_ref_unsafe("HEAD", head_sha1, 1, NULL))
            return error(_("Could not resolve HEAD commit\n"));

        head_commit = lookup_commit(head_sha1);
        if (!head_commit || parse_commit(head_commit))
            return error(_("could not parse commit %s\n"),
                     sha1_to_hex(head_commit->object.sha1));

In the last line quoted above, head_commit may be NULL, and it is dereferenced in the call to sha1_to_hex. Assuming lookup_commit(head_sha1) can return NULL, this function will segfault.

Introduced here: https://github.com/gitster/git/commit/b27cfb0#sequencer.c

Mike