Fix PrettierAsync ignored file contents mangling

[SamHowie]

Summary
This pull request fixes #140 where the PrettierAsync command causes the buffer contents to become garbled if the file is ignored by prettier.

It was found to be an issue with more recent versions of prettier that echo the stdin[1] of ignored files back to stdout[2].

1. https://github.com/prettier/prettier/blob/fb74dc54c337b92e2be6dfffe147ab42143e4e48/src/cli/util.js#L313
2. https://github.com/prettier/prettier/blob/fb74dc54c337b92e2be6dfffe147ab42143e4e48/src/cli/util.js#L126

Unfortunately, Vim channels do not like this very much. Vim writes NUL bytes to Prettier's stdin to indicate line endings. However, Prettier is echoing them back and then Vim is having trouble reading them back from what it thinks is a string, leading to undefined behaviour.

A String in Vim cannot contain NUL bytes. To send or receive NUL bytes read or write from a buffer. See |in_io-buffer| and |out_io-buffer|.

https://github.com/vim/vim/blob/master/runtime/doc/channel.txt#L407

The work around is to do what the vim documentation suggests and let the job write to a hidden buffer. This doesn't seem to be effected by the segmentation fault issue the input buffer was causing.

I also was having a weird rendering issue if the file contents had not changed. I added a redraw! to that code path and it fixed it. I am not sure if this issue was pre-existing.

Test Plan

0. Make sure you have the latest Prettier installed
1. Create a new directory
2. Create a file called foo.js
3. Create a .prettierignore file
4. Add foo.js to the .prettierignore file
5. Add several lines of text to the foo.js file
6. Save the foo.js file
7. Notice the file contents are not garbled on the last line

[SamHowie]

I have checked this on Vim 8. @mitermayer could you check on Vim 7.4 and NeoVim?

[mitermayer]

Hi @SamHowie,

Thanks for looking into this! This is great, will do some sanity testing and comment on this issue