t0060: verify that basename() and dirname() work as expected

t0060: verify that basename() and dirname() work as expected

Unfortunately, some libgen implementations yield outcomes different
from what Git expects. For example, mingw-w64-crt provides a basename()
function, that shortens `path0/` to `path`!

So let's verify that the basename() and dirname() functions we use
conform to what Git expects.

Derived-from-code-by: Ramsay Jones <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>