tests/test-backwards-remove

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.

#!/bin/sh -x
hg init
echo This is file a1 > a
hg add a
hg commit -m "commit #0" -d "0 0"
ls
echo This is file b1 > b
hg add b
hg commit -m "commit #1" -d "0 0"
hg co 0
# B should disappear
ls