Add core copy detection algorithm This adds findcopies, which detects merge-relevant copies between files in a pair of manifests back to the merge ancestor. While the merge code invokes the copy detection routine, it does not yet use the result.

adding base
abort: cannot delete applied patch c
Now at: b
a
b
a
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Now at: a
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1 New patch: a
0 base
abort: patch e not at base
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4 New patch: f
3 New patch: e
2 New patch: d
1 New patch: a
0 base