fast-import: filemodify after M 040000 <tree> "" crashes

fast-import: filemodify after M 040000 <tree> "" crashes

Until M 040000 <tree> "" syntax was introduced in commit 2794ad5
(fast-import: Allow filemodify to set the root, 2010-10-10), it
was impossible for the root entry to refer to an unloaded tree.
Update various functions to take that possibility into account.
Otherwise

	M 040000 <tree> ""
	M 100644 :1 "foo"

and similar commands (using D, C, or R after resetting the root
tree) segfault.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>