win32: override `fspathcmp()` with a directory separator-aware version

win32: override `fspathcmp()` with a directory separator-aware version

On Windows, the backslash is the directory separator, even if the
forward slash can be used, too, at least since Windows NT.

This means that the paths `a/b` and `a\b` are equivalent, and
`fspathcmp()` needs to be made aware of that fact.

Note that we have to override both `fspathcmp()` and `fspathncmp()`, and
the former cannot be a mere pre-processor constant that transforms calls
to `fspathcmp(a, b)` into `fspathncmp(a, b, (size_t)-1)` because the
function `report_collided_checkout()` in `unpack-trees.c` wants to
assign `list.cmp = fspathcmp`.

Also note that `fspatheq()` does _not_ need to be overridden because it
calls `fspathcmp()` internally.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>