[Buffers] Use reltimefloat(reltime()) instead of localtime() (#152)

* Nanoseconds instead of seconds for buffer timestmaps

Fzf :Buffers uses a dictionary where it stores the last time accessed of
the current buffers. This is used to sort the buffers.
The timestamp is retrieved from a call to localtime() which returns the
current second the buffer is accessed. However, fzf functions in such a
way that before it enters the newly selected buffer, it enters the
current buffer. Sometimes this causes the last two buffers to have the
same timestamp. Repeatedly switching between two buffers yields
inconsistent results.
This commit fixes the issue by using seconds and microseconds from the
bash call `date`. The buffers are now guaranteed to be sorted by last
accessed.

* Use reltime() instead of linux date

This solution is better for compatability reasons.

* Remove trailing whitespace

autoload/fzf/vim.vim

@@ -479,7 +479,8 @@ endfunction
 
 function! s:sort_buffers(...)
   let [b1, b2] = map(copy(a:000), 'get(g:fzf#vim#buffers, v:val, v:val)')
-  return b1 - b2
+  " Using minus between a float and a number in a sort function causes an error
+  return b1 > b2 ? 1 : -1
 endfunction
 
 function! fzf#vim#buffers(...)

plugin/fzf.vim

@@ -112,7 +112,7 @@ endif
 let g:fzf#vim#buffers = {}
 augroup fzf_buffers
   autocmd!
-  autocmd BufWinEnter,WinEnter * let g:fzf#vim#buffers[bufnr('')] = localtime()
+  autocmd BufWinEnter,WinEnter * let g:fzf#vim#buffers[bufnr('')] = reltimefloat(reltime())
   autocmd BufDelete * silent! call remove(g:fzf#vim#buffers, expand('<abuf>'))
 augroup END