[RFC] diff: Send the text of the comparison through stdin.

Slight performance increase of `:diff[update|split]` in using `os_system()` to send the contents of the buffer. Here's my benchmark:
```vim
function! s:diff()
  edit nvim/diff.c 
  vert diffsplit nvim/eval.c " Test for two completely different files.
  new | only
  edit nvim/diff.c
  vert diffsplit nvim/diff.master " Two similar files.
endfunction

command! Do call s:diff()
```
And the results zooming in on the `diffsplit` lines:
```
branch  total (s)   self (s)
mastr   0.597878   0.269030    completely different
this    0.441984   0.233965   completely different

mastr   0.325608   0.111435   similar
this    0.256152   0.107692   similar
```
so not the most exciting results in the world. Note: I found I got the biggest performance increase when I had CPU-intensive processes running in the background. I attempted various things such as running the `diff` program as a job, using a read callback, and using `os_system()` to get the output instead of using redirection, but I actually found these made it slower. (In case anyone's curious to test it, here's my branch for running `diff` in a job: https://github.com/splinterofchaos/neovim/tree/diff-job-in)