diriter: don't double '/' on Windows

The canonical directory path of the root directory of a volume on
windows already ends in a slash (eg, `c:/`).  This is true only
at the volume root.  Do not add a slash to paths in this case.

src/path.c

@@ -1166,7 +1166,11 @@ static int diriter_update_paths(git_path_diriter *diriter)
 	diriter->path[path_len-1] = L'\0';
 
 	git_buf_truncate(&diriter->path_utf8, diriter->parent_utf8_len);
-	git_buf_putc(&diriter->path_utf8, '/');
+
+	if (diriter->parent_utf8_len > 0 &&
+		diriter->path_utf8.ptr[diriter->parent_utf8_len-1] != '/')
+		git_buf_putc(&diriter->path_utf8, '/');
+
 	git_buf_put_w(&diriter->path_utf8, diriter->current.cFileName, filename_len);
 
 	if (git_buf_oom(&diriter->path_utf8))