dir_inside_of(): handle directory separators correctly

On Windows, both the forward slash and the backslash are directory
separators. Which means that `a\b\c` really is inside `a/b`. Therefore,
we need to special-case the directory separators in the helper function
`cmp_icase()` that is used in the loop in `dir_inside_of()`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

dir.c

@@ -3142,6 +3142,8 @@ static int cmp_icase(char a, char b)
 {
 	if (a == b)
 		return 0;
+	if (is_dir_sep(a))
+		return is_dir_sep(b) ? 0 : -1;
 	if (ignore_case)
 		return toupper(a) - toupper(b);
 	return a - b;