t9300: test filedelete command

Add new fast-import test series for filedelete command.

Signed-off-by: Maxim Bublis <satori@yandex-team.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
fast-import: fix segfault in store_tree()

Branch tree is NULLified by filedelete command if we are trying
to delete root tree. Add sanity check and use load_tree() in that case.

Signed-off-by: Maxim Bublis <satori@yandex-team.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>