WIP: fixing filelog recording with copy overwrite
These changesets introduce test case for situation where a new file overwrite an older one during a merge.
The current implementation (in the first commit) is pretty awful and we would like something cleaner.
The situation is properly detected at merge time, so it is mostly a matters of carrying this information to commit time. To do so, adding a new optional entry in the merge state (at merge time) and reading later (at commit time). Seems like a good option.