The root item's path is ".", and the path of a file at top level is
"./file". When using GetPath, this gives us "." and "file",
respectively, and isDescendentOfSelectedCommitFileNodes would return
false for these.
Working with the internal paths (i.e. without stripping the leading
"./") fixes this.
There is no known breakage that is caused by this, that's why I'm not
adding an integration test that demonstrates a bug.
Equivalent to the change that was made to isDescendentOfSelectedNodes in
files_controller.go in 302b621b68.