Commit 5edde510 (fast-import: filemodify after M 040000 <tree> ""
crashes, 2010-10-17) taught fast-import to load trees from the
object db as needed when it is time to access them.

But it went too far.  In change_note_fanout(), an empty,
not-loaded tree is not meant to destroy notes, so calling
load_tree() at that point is exactly the wrong thing to do.

Kudos to Johan Herland for t9301, which caught this failure.
Reported-by: NThomas Rast <trast@student.ethz.ch>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>