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

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 <redacted>