exclude .bzrignore when exporting

[Jamie Wilkinson]

Hi!

I think it'd be worthwhile to exclude the .bzrignore file when exporting to
tarball, because the file doesn't have any use outside of a bzr working
tree.

I've got a simple patch to bzrlib/tree.py that does this, 
http://anchor.net.au/~jaq/bzr.jaq/
revno 1391

I was going to add to the blackbox test suite too, but wasn't sure if it
should go in the export tests, or the ignore tests.