mergecopies: reuse ancestry context when traversing file history (issue4537) Merge copies is traversing file history in search for copies and renames. Since 3.3 we are doing "linkrev adjustment" to ensure duplicated filelog entry does not confuse the traversal. This "linkrev adjustment" involved ancestry testing and walking in the changeset graph. If we do such walk in the changesets graph for each file, we end up with a 'O(<changesets>x<files>)' complexity that create massive issue. For examples, grafting a changeset in Mozilla's repo moved from 6 seconds to more than 3 minutes. There is a mechanism to reuse such ancestors computation between all files. But it has to be manually set up in situation were it make sense to take such shortcut. This changesets set this mechanism up and bring back the graph time from 3 minutes to 8 seconds. To do so, we need a bigger control on the way 'filectx' are instantiated during each 'checkcopies' calls that 'mergecopies' is doing. We add a new 'setupctx' that configure and return a 'filectx' factory. The function make sure the ancestry context is properly created and the factory make sure it is properly installed on returned 'filectx'.