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.
#!/bin/sh
echo % init
hg init
echo % commit
echo 'a' > a
hg ci -A -m test -u nobody -d '1 0'
echo % annotate -c
hg annotate -c a
echo % annotate -d
hg annotate -d a
echo % annotate -n
hg annotate -n a
echo % annotate -u
hg annotate -u a
echo % annotate -cdnu
hg annotate -cdnu a