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