[MERGE] Fix for bug 183831

[Geoff Bache]

Hi all,

OK, here comes a new patch, which changes only tree_files_for_add, and
adds two blackbox tests around symbolic link usage for "bzr add". It
isn't beautiful but it does fix the bug and doesn't break anything
else. I gave up trying to reconcile the interface of
mutabletree.smart_add, there were just too many unit tests that assume
the current one and figuring out what they're all supposed to do and
how to migrate them is going to take more time than I have.

So the current setup converts the correct relative path to an absolute
path all ready for smart_add to convert it back again. If one of the
gurus has time for some refactoring I'd suggest this code is an
excellent candidate :)

Regards,
Geoff Bache