backup: fix issue when the backup end up in a different directory Because of store encoding, we might end up with the backup in a different directory than the initial copy (for example if the backup path make it cross the 120 char limit). This can create crash, especially since 6.4 where backup are used during revlog split. Making sure the directory exists fixes these crash We added a test covering this case. Strictly speaking, this has always been broken, however the new code in 6.4 triggers it more easily.