meldbuffer: Set buffer label in reset_buffer()

Retain the original label if it's valid.  If it's not, set the label
equal to the buffer filename.

Without this change the labels in file diffs default to "unnamed"
values.

Signed-off-by: Peter Tyser <ptyser@gmail.com>
tree.py: Fix treeview_search_cb() argument mismatch

When trying to find a file in dirdiff or vcview mode the following
error would be displayed:
  TypeError: treeview_search_cb() takes exactly 5 arguments (6 given)

Add an unused "data" argument to treeview_search_cb() to silence the
warning and allow file searching to work.

Signed-off-by: Peter Tyser <ptyser@gmail.com>