t0027: do not use an empty string as a pathspec element

In an upcoming update, we will finally make an empty string illegal as an element in a pathspec; it traditionally meant the same as ".", i.e. include everything, so update this test that passes "" to pass a dot instead. At this point in the test sequence, there is no modified path that need to be further added before committing; the working tree is empty except for .gitattributes which was just added to the index. So we could instead pass no pathspec, but this is a conversion more faithful to the original. Signed-off-by: Junio C Hamano <gitster@pobox.com>