win32/file.c: direct conversion

* win32/file.c (user_length_in_path): count user name length in
  path.

* win32/file.c (append_wstr): append WCHAR string to Ruby string
  directly without an intermediate buffer, if possible.

* win32/file.c (rb_file_expand_path_internal): use above functions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47118 b2dd03c8-39d4-4d8f-98ff-823fe69b080e