[MERGE] Improve "Path(s) are not versioned" error reporting for some commands

[Benoît PIERRE]

Without this patch, 'bzr revert build NEWS' in the Bazaar source directory yields:

  bzr: ERROR: Path(s) are not versioned: build

while 'bzr diff build NEWS' reports:

  bzr: ERROR: Path(s) are not versioned: build NEWS

This patch changes iter_changes in bzrlib/workingtree_4.py to only show the
files which are actually not versioned, and fixes (at least) the commands
shelve and diff.

I honestly did not go through the testsuite because even without
this patch I get quite a few errors and some hangups during
selftest.

-- 
A: Because it destroys the flow of conversation.
Q: Why is top posting dumb?
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