[vim] Fix double path separator issue on Windows

Fix https://github.com/junegunn/fzf.vim/issues/1141

plugin/fzf.vim

@@ -283,7 +283,8 @@ function! s:common_sink(action, lines) abort
     let cwd = exists('w:fzf_pushd') ? w:fzf_pushd.dir : expand('%:p:h')
     for item in a:lines
       if item[0] != '~' && item !~ (s:is_win ? '^[A-Z]:\' : '^/')
-        let item = join([cwd, item], (s:is_win ? '\' : '/'))
+        let sep = s:is_win ? '\' : '/'
+        let item = join([cwd, item], cwd[len(cwd)-1] == sep ? '' : sep)
       endif
       if empty
         execute 'e' s:escape(item)