fast-import: properly fanout notes when tree is imported

fast-import: properly fanout notes when tree is imported

In typical uses of fast-import, trees are inherited from a parent
commit. In that case, the tree_entry for the branch looks like:

  .versions[1].sha1 = $some_sha1
  .tree = <tree structure loaded from $some_sha1>

However, when trees are imported, rather than inherited, that is not the
case. One can import a tree with a filemodify command, replacing the
root tree object.

e.g.
  "M 040000 $some_sha1 \n"

In this case, the tree_entry for the branch looks like:

  .versions[1].sha1 = $some_sha1
  .tree = NULL

When adding new notes with the notemodify command, do_change_note_fanout
is called to get a notes count, and to do so, it loops over the
tree_entry->tree, but doesn't do anything when the tree is NULL.

In the latter case above, it means do_change_note_fanout thinks the tree
contains no notes, and new notes are added with no fanout.

Interestingly, do_change_note_fanout does check whether subdirectories
have a NULL .tree, in which case it uses load_tree(). Which means the
right behaviour happens when using the filemodify command to import
subdirectories.

This change makes do_change_note_fanount call load_tree() whenever the
tree_entry it is given has no tree loaded, making all cases handled
equally.

Signed-off-by: Mike Hommey <mh@glandium.org>
Reviewed-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>