tests/test-dirstatedirs.out

ancestor: improve docstring

The term "least common ancestor" is highly ambiguous.

adding a/b/c/d/x
adding a/b/c/d/y
adding a/b/c/d/z
moving a/b/c/d/x to z/b/c/d/x
moving a/b/c/d/y to z/b/c/d/y
moving a/b/c/d/z to z/b/c/d/z