copytrace: move the default copytracing algorithm in a new function
We are going to introduce a new fast heuristic based copytracing algorithm, so
lets make mergecopies the function which decides which algorithm to go with and
then calls the related function.
While I was here, I add a line in test-copy-move-merge.t saying its a test
related to the full copytracing algorithm.
Differential Revision: https://phab.mercurial-scm.org/D622
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