Renaming a file inside the same directory shows it with its full path in the
tree view, which isn't what we want. We'll fix this in the next commit.
Also adding a few other test cases for moving files; they show that the display
of moved files in tree view isn't ideal. For example, moving file1 from top
level into dir shows it as "R file1 → file1", which isn't distinguishable from
renaming file1 inside dir. I suppose what we would like to have here is
"R ../file1 → file1" or something, but I'll leave that for the future; here I
only want to fix the regression that was introduced with the root item PR.
