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>

dir.c

@@ -95,7 +95,7 @@ int count_slashes(const char *s)
 	return cnt;
 }
 
-int fspathcmp(const char *a, const char *b)
+int git_fspathcmp(const char *a, const char *b)
 {
 	return ignore_case ? strcasecmp(a, b) : strcmp(a, b);
 }
@@ -105,7 +105,7 @@ int fspatheq(const char *a, const char *b)
 	return !fspathcmp(a, b);
 }
 
-int fspathncmp(const char *a, const char *b, size_t count)
+int git_fspathncmp(const char *a, const char *b, size_t count)
 {
 	return ignore_case ? strncasecmp(a, b, count) : strncmp(a, b, count);
 }