t: restrict `is_hidden` to be called only on Windows
The function won't work anywhere else, so let's mark it as an explicit
bug if it is called on a non-Windows platform.
Let's also rename the function to avoid cluttering the global namespace
with an overly-generic function name.
While at it, we also fix the code comment above that function: the
lower-case `windows` refers to something different than `Windows`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>