unchecked uses of strdup

There are pretty many uses of strdup in git's sources. Here's one that can cause trouble if it ever returns NULL:

    [from fsck-objects.c]
    static int fsck_head_link(void)
    {
            unsigned char sha1[20];
            const char *git_HEAD = strdup(git_path("HEAD"));
            const char *git_refs_heads_master = resolve_ref(git_HEAD, sha1, 1);

The problem is that resolve_ref does an unconditional `stat' on the parameter corresponding to the maybe-NULL git_HEAD.

One solution is to change such uses of strdup to uses of xstrdup.