Close #1874

e.g.

  let g:fzf_layout = { 'window': { 'width': 0.4, 'height': 1, 'xoffset': 0, 'border': 'right' } }
  let g:fzf_layout = { 'window': { 'width': 0.4, 'height': 1, 'xoffset': 1, 'border': 'left' } }
  let g:fzf_layout = { 'window': { 'width': 1, 'height': 0.5, 'yoffset': 1, 'border': 'top' } }
  let g:fzf_layout = { 'window': { 'width': 1, 'height': 0.5, 'yoffset': 0, 'border': 'bottom' } }

Revert the change introduced in #552. It seems that the startup time
difference between bash and fish is not much of an issue now.

  > time bash -c 'date'
  Thu Feb 13 21:15:03 KST 2020

  real    0m0.008s
  user    0m0.003s
  sys     0m0.003s

  > time fish -c 'date'
  Thu Feb 13 21:15:05 KST 2020

  real    0m0.014s
  user    0m0.007s
  sys     0m0.006s

When we explicitly *pipe* $FZF_DEFAULT_COMMAND instead of making fzf
internally start the process ($FZF_DEFAULT_COMMAND | fzf), fzf may hang
if the input process doesn't quickly process SIGPIPE and abort.

Also, fzf#vim#grep temporarily swaps $FZF_DEFAULT_COMMAND instead of
setting 'sink' so fzf can kill the default command on 'reload'.

https://github.com/junegunn/fzf.vim/issues/927

However, because of the "pipe conversion", the trick wasn't working as
expected.

https://github.com/junegunn/fzf.vim/blob/467c3277884240f7b5430f8f4d600e3415c38f3b/autoload/fzf/vim.vim#L720-L726

We can go even further and always set $FZF_DEFAULT_COMMAND instead of
piping source command.

Credits to @jiangjianshan

Close #1856
Close #1857

Close https://github.com/junegunn/fzf.vim/issues/942

Makes scanning 300x faster on Windows

e.g.
  let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }

Based on the code from https://github.com/junegunn/fzf.vim/issues/821#issuecomment-581273191
by @lacygoill.

Update copyright year to 2020 and change to multi-year format.

Fix #1637 

Tested on urxvt.
Fix #1799.

Enables the right bindings when switching between editing modes.

Fix #1795

Truecolor does not work on default Windows terminal.
It is a problem in neovim GUIs.

https://github.com/sainnhe/edge/issues/5#issuecomment-565748240

* Add MSYS2 support as a vim plugin

Add &shellcmdflag and TERM environment variable treatment.

- Make &shellcmdflag `/C` when &shell turns into `cmd.exe`
- Delete %TERM% environment variable before fzf execution

* Change shellescape default value depending on s:is_win flag

* Make TERM environment empty only when gui is running

* Stop checking &shell in fzf#shellescape function

This funcion's behavior is controlled by only if it is Windows or not.
So there is no need to check &shell.

* Take neovim into consideration when to set shellcmdflag

* Add &shellxquote control

Close #1776

Backport https://github.com/junegunn/vim-plug/pull/913