It was discovered that when restic is instructed to save `/`, the tree
structures in the repository contain an invalid node.
When saving the dir `/home/user`, the following structure is created:
snapshot
-> tree
nodes: ["user"]
[...]
When the root directory `/` is saved, the structure is as follows:
snapshot
-> tree
nodes: ["/"]
[...]
This behavior is caused by the walker in pipe.go sending a node with the
name "." to the archiver, so this commit adds a test for invalid node
names.
