vim-patch:8.2.{2518,2520,3572,3588}: 'listchars' (and 'fillchars'?) fixes

vim-patch:8.2.3627: difficult to know where the text starts in a window

Problem:    difficult to know where the text starts in a window. (Sergey
            Vlasov)
Solution:   Add the "textoff" entry in the result of getwininfo().
            (closes vim/vim#9163)
https://github.com/vim/vim/commit/cdf5fdb2948ecdd24c6a1e27ed33dfa847c2b3e4

Fix indent in Test_getbufwintabinfo().

vim-patch:8.2.2922,8.2.3639

Errors were being caused by invalid buffers being kept around in
diagnostic_cache, so add a metatable to diagnostic_cache which attaches
to new buffers in the cache, removing them after they are invalidated.

Closes #16391.

Co-authored-by: Gregory Anders <8965202+gpanders@users.noreply.github.com>

vim-patch:8.2.{3468,3617,3618,3622}: some other CWD related patches

Problem:    ci" finds following string but ci< and others don't.
Solution:   When not inside an object find the start. (Connor Lane Smit,
            closes vim/vim#8670)
https://github.com/vim/vim/commit/b9115da4bec5e6cfff69da85cc47c42dd67e42e4

Problem:    On a narrow screen ":hi" output is confusing.
Solution:   Insert a space between highlight group name and "xxx". (Masato
            Nishihaga, closes vim/vim#4599)
https://github.com/vim/vim/commit/548be7f126dc57ca861cb26dc6492c3b2a9e2c99

Problem:    getcompletion() always passes zero as position to custom
            completion function.
Solution:   Pass the pattern length. (closes vim/vim#9173)
https://github.com/vim/vim/commit/4785fe02bba14c4e0aede0fa425ca790a1cb98d7

Co-authored-by: ii14 <ii14@users.noreply.github.com>