Goes well with --height

Due to performance consideration, FuzzyMatchV2 does not return the exact
positions of the matching characters by default. However, the ommission
caused `--tiebreak=begin` to produce inaccurate result in some cases.

  (echo baz foo bar; echo foo bar baz) | fzf --tiebreak=begin -fbar | head -1

  # Expected: foo bar baz
  # Actual:   baz foo bar

This commit fixes the problem by using the end offset which is
guaranteed to be correct.

https://github.com/junegunn/go-runewidth/pull/1

/cc @joshuarubin

Exposing the `b:fzf` variable will be useful to get information about
which command is being executed on the current fzf window. With that,
now, it's possible to use the current command name on the statusline:

```viml
au User FzfStatusLine call <SID>fzf_statusline()

function! s:fzf_statusline()
  let fzf_cmd_name = get(b:fzf, 'name', 'FZF')
  let &l:statusline = '> '.fzf_cmd_name
endfunction
```

https://github.com/junegunn/fzf.vim/issues/296

- Move cursor to the top-left corner when starting a command in
  alternate screen
- Fix cursor position when returning to alternate screen when fzf is
  running in full screen mode

Close #825

Close #823

Close #788

Related #822

Close #822

Close #821

Close #820

Close #497

Close #818

- Remove unnecessary scrolling
- Allow us to use `--height 100%` under Neovim terminal for 24-bit colors

Related:
- #789
- https://github.com/neovim/neovim/issues/4151

Also revert the workaround in Vim plugin introduced in fa7c8977.

Related: #814

Close #814

Not grouping commands to avoid errors on non-standard shells.