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