untracked: fix detection of uname(2) failure

According to POSIX specification uname(2) must return -1 on failure
and a non-negative value on success.  Although many implementations
do return 0 on success it is valid to return any positive value for
success.  In particular, Solaris returns 1.
Signed-off-by: NCharles Bailey <cbailey32@bloomberg.net>
Reviewed-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

dir.c

@@ -1801,7 +1801,7 @@ static const char *get_ident_string(void)
 
 	if (sb.len)
 		return sb.buf;
-	if (uname(&uts))
+	if (uname(&uts) < 0)
 		die_errno(_("failed to get kernel name and information"));
 	strbuf_addf(&sb, "Location %s, system %s %s %s", get_git_work_tree(),
 		    uts.sysname, uts.release, uts.version);