copies: add a config to limit the number of candidates to check in heuristics The heuristics algorithm find possible candidates for move/copy and then check whether they are actually a copy or move. In some cases, there can be lot of candidates possible which can actually slow down the algorithm. This patch introduces a config option `experimental.copytrace.movecandidateslimit` using which one can limit the candidates to check. The limit defaults to 100. Thanks to Yuya for suggesting to skip copytracing for that file with a warning. Differential Revision: https://phab.mercurial-scm.org/D987