The checksums are not the same at each round, trying to determine why

-> .svn was not ignored during the tree hash computation
-> tree hash function migrated from swh.loader.dir to swh.model
+ updated to deal with possible folders to ignore

cf. swh-model rDMOD2a0107ee2bcdc37eb127a3c7c7b4f17e82d54e9f + rDMOD5ca1bda3aa295da6bcd3200b6ba335e56da3c9ff