Merge branch 'da/difftool' into maint

The code to prepare the working tree side of temporary directory
for the "dir-diff" feature forgot that symbolic links need not be
copied (or symlinked) to the temporary area, as the code already
special cases and overwrites them.  Besides, it was wrong to try
computing the object name of the target of symbolic link, which may
not even exist or may be a directory.

* da/difftool:
  difftool: ignore symbolic links in use_wt_file