Test pipe walker for invalid paths
Test pipe walker for invalid paths

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.

stuertz/restic,jayme-github/restic,bchapuis/restic,restic/restic,kurin/restic,middelink/restic,jayme-github/restic,mappu/restic,byo/restic,fawick/restic,middelink/restic,stuertz/restic,restic/restic,fawick/restic,ar-jan/restic,ar-jan/restic,byo/restic,arithmetric/restic,kurin/restic,ckemper67/restic