Resolve paths to avoid duplicates in UltiSnipsEdit (#1440)

Use realpath to get the actual path so symlink directories are counted
as equal and excluded from the set of output.

When dotfiles are symlinked:

> dir c:\users\idbrii\.vim
...
2020-12-08  04:01 PM    <SYMLINKD>     .vim [C:\dotfiles\vim]

`:UltiSnipsEdit!` outputs:

    1: C:\Users\idbrii/.vim/bundle/david-snippets/UltiSnips/fugitive.snippets
  * 2: c:\dotfiles\vim\bundle\david-snippets\ultisnips\all.snippets
  * 3: c:\dotfiles\vim\bundle\david-snippets\ultisnips\all\shebang.snippets
  * 4: c:\dotfiles\vim\bundle\snippets\ultisnips\all.snippets
  * 5: c:\dotfiles\vim\bundle\project\ultisnips\all.snippets
  * 6: c:\users\idbrii\.vim\bundle\david-snippets\ultisnips\all.snippets
  * 7: c:\users\idbrii\.vim\bundle\david-snippets\ultisnips\all\shebang.snippets
    8: c:\users\idbrii\.vim\ultisnips\all.snippets
    9: c:\users\idbrii\.vim\ultisnips\fugitive.snippets

2,6 and 3,7 are duplicates.

This fixes the same issue on linux with similar symlink setup:
  $ ls -l ~/.vim
  lrwxrwxrwx 1 idbrii idbrii 49 Jan 24  2021 /home/idbrii/.vim -> /home/idbrii/dotfiles/vim