Merge branch 'da/difftool'

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