Fix :Tags command tagfile relative path

Since fzf will execute the source command in the tagfile's directory, we
can't give the path of the tagfile relative to the current working
directory. Instead give just the filename.

autoload/fzf/vim.vim

@@ -437,7 +437,7 @@ function! fzf#vim#tags(...)
     let copt = '--ansi '
   endif
   call s:fzf({
-  \ 'source':  proc.shellescape(tagfile),
+  \ 'source':  proc.shellescape(fnamemodify(tagfile, ':t')),
   \ 'dir':     fnamemodify(tagfile, ':h'),
   \ 'options': copt.'+m --tiebreak=begin --prompt "Tags> "'.s:expect(),
   \ 'sink*':   function('s:tags_sink')}, a:000)