merge: use vfs methods for I/O All I/O is supposed to be performed via vfs instances so filesystems can be abstracted. The previous commit ported the old code in purge, which didn't go through the vfs layer. This commit ports the purge code to use the vfs layer. The vfs layer didn't have a method to remove a single directory, so it was added as part of implementing this. Differential Revision: https://phab.mercurial-scm.org/D4478

merge: move purge logic from extension Working directory purging feels like functionality that should be in core rather than in an extension. This commit effectively moves the core purging logic from the purge extension to merge.py. Code was refactored slightly. Rather than deal with printing in this function, the function is a generator of paths and the caller can worry about printing. Differential Revision: https://phab.mercurial-scm.org/D4477

tests: stabilize test-removeemptydirs.t on Windows