copies: refactor symmetricdifference as _findlimit
We only need to track the lowest revision seen, which makes things simpler.
Checked out revision 0.
A a
Adding a
Transmitting file data .
Committed revision 1.
% initial roundtrip
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0 added a
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% second roundtrip should do nothing
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% new hg rev
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1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% echo hg to svn
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0 b
% svn back to hg should do nothing
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% hg back to svn should do nothing
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