Fix performance regression in addremove command. When I rewrote addremove, I lazily put a call to repo.changes in, which was unnecessary and slow. This is a new rewrite, preserving the file name behaviour, but replacing the call to repo.changes with a walk, which is much cheaper, and avoids calls to os.stat on all but files that have probably been deleted.

#header#
# HG changeset patch
# User #author#
# Date #date|date#
# Node ID #node#
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