copytrace: move fast heuristic copytracing algorithm to core copytrace extension in fb-hgext has a heuristic implementation of copy tracing which is faster than the current copy tracing. The heuristic limits the search of copies to just files that are either: 1) Renames in the same directory 2) Moved to other directory with same name The default copytrace implementation is very slow as it finds all the new files that were added from merge base up to the head commit and for each file it checks whether it this was copied or moved version of a different file. Stash@fb did analysis for the above heuristics on the fb repo and found that among 2,443,768 moves/copies there are only 32,234 moves/copies which does not fall under the above heuristics which is approx. 0.013 of total copies. This patch moves the heuristics algorithm under config `experimental.copytrace=heuristics`. While moving fbext to core, this patch removes couple of less useful config options named `sourcecommitlimit` and `maxmovescandidatestocheck`. Tests are also added for the heuristics algorithm, which are basically copied from fbext/tests/test-copytrace.t. The tests follow a pattern creating a server repo and then cloning to a local repo to create public and draft changesets, the distinction which will be useful in upcoming patches. After this patch `experimental.copytrace` has the following behaviour: 1) `off`: turns off copytracing 2) `heuristics`: use the heuristic algorithm added in this patch. 3) everything else: use the full copytracing algorithm .. feature:: A new fast heuristic algorithm for copytracing which assumes that the files moves are either:: 1) Renames in the same directory 2) Moves in other directories with same names You can use this algorithm by setting `experimental.copytrace=heuristics`. Differential Revision: https://phab.mercurial-scm.org/D623