commit: don't be fooled by ita entries when creating initial commit

commit: don't be fooled by ita entries when creating initial commit

ita entries are dropped at tree generation phase. If the entire index
consists of just ita entries, the result would be a a commit with no
entries, which should be caught unless --allow-empty is specified. The
test "!!active_nr" is not sufficient to catch this.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>