compositor: avoid transmitting invalid lines on double scroll

This happens in an operation which both increases topline and also
inserts new lines somewhere in the remaining are. So before drawing any
line, win_update() is performing two grid_scroll operations.

===
A
B
C
D
E^
F
===

Consider that new line will be inserted after line E and screen also
scrolled up to line C. First the topline will be adjusted (x is the
scrolling region, ! invalid/empty space created by the scroll):

===
C   x
D   x
E^  x
F   x
!   x
!   x
===

and then space is inserted for the new line

===
C
D
E^
!   x
F   x
!   x
===

The problem is that we are now assuming that any invalid area ! created
by a scroll is filled with actual contents (by win_line etc) before it
is scrolled again. But in this case the last invalid line ! gets
scrolled. Ideally we should make win_update smarter and just scroll
valid lines for the later scroll (it is just wasteful to scroll the
larger area anyway), but for the 0.4 releasejust make
the compositor ignore such an invalid line (as it will get overdrawn
anyway later).

Set the environment variables before changing $HOME
so that python,ruby can find user modules not in $HOME.

Reference:
- https://github.com/neovim/neovim/commit/7be7ec98a22451ec75ce5eb9284fa1722b53c697
- https://github.com/neovim/neovim/issues/10270#issuecomment-531516948

Problem:    Ruby converts v:true and v:false to a number.
Solution:   Use Qtrue and Qfalse instead. (Masataka Pocke Kuwabara,
            closes vim/vim#3259)
https://github.com/vim/vim/commit/d84b26a03b13cd816d80ff32b61e8de740d499ce

nvim does not support v:none.

Given the other type names "jumps", "vars", etc., the name "buflist"
is somewhat unintuitive.

Since the parameter is already non-primitive, make it an `opts` map
instead of just a list, in case we want to extend it later.

vim-patch:8.0.{1134,1148},8.1.{212,289,2028}
fix #10270

fix #8428

Problem:    Preferred cursor column not set in interfaces.
Solution:   Set w_set_curswant when setting the cursor. (David Hotham,
            closes vim/vim#3060)
https://github.com/vim/vim/commit/53901442f37a59e5495165f91db5574c0b43ab04

This contains test_ruby changes only.
test_python changes were ported before.