Problem:  When the quickfix buffer has been modified an autocommand
          may invalidate the undo stack (kawarimidoll)
Solution: When clearing the quickfix buffer, also wipe the undo stack

fixes: vim/vim#13905
closes: vim/vim#13928

https://github.com/vim/vim/commit/f0d3d4a42657dca996e790aa829de3c6be7fdb63

Co-authored-by: Christian Brabandt <cb@256bit.org>

Problem:  Visual highlight hard to read with 'termguicolors'
          (Maxim Kim)
Solution: Set Visual GUI foreground to black (with background=light)
          and lightgrey (with background=dark)
          (Maxim Kim)

fixes: vim/vim#14024
closes: vim/vim#14025

https://github.com/vim/vim/commit/34e4a05d02a016fe230495be8f6c60ddd56f9567

Co-authored-by: Maxim Kim <habamax@gmail.com>

Problem:  upper-case of ß should be U+1E9E (CAPITAL LETTER SHARP S)
          (fenuks)
Solution: Make gU, ~ and g~ convert the U+00DF LATIN SMALL LETTER SHARP S (ß)
          to U+1E9E LATIN CAPITAL LETTER SHARP S (ẞ), update tests
          (glepnir)

This is part of Unicode 5.1.0 from April 2008, so should be fairly safe
to use now and since 2017 is part of the German standard orthography,
according to Wikipedia:
https://en.wikipedia.org/wiki/Capital_%E1%BA%9E#cite_note-auto-12

There is however one exception: UnicodeData.txt for U+00DF
LATIN SMALL LETTER SHARP S does NOT define U+1E9E LATIN CAPITAL LETTER
SHARP S as its upper case version. Therefore, toupper() won't be able
to convert from lower sharp s to upper case sharp s (the other way
around however works, since U+00DF is considered the lower case
character of U+1E9E and therefore tolower() works correctly for the
upper case version).

fixes: vim/vim#5573
closes: vim/vim#14018

https://github.com/vim/vim/commit/bd1232a1faf56b614a1e74c4ce51bc6e0650ae00

Co-authored-by: glepnir <glephunter@gmail.com>

Problem:  Visual hl wrong when it ends before multibyte 'showbreak'.
          (lacygoil)
Solution: Use vcol_sbr instead of adding n_extra.
          (zeertzjq)

fixes: vim/vim#11272
closes: vim/vim#13996

https://github.com/vim/vim/commit/df23d7f4bd7546f3152ea003856525591218565b

Bug doesn't apply to Nvim.

Problem:  Redrawing can be improved when inserting/deleting lines with 'number'.
Solution: Only redraw the number column of lines below changed lines.
          Add a test as this wasn't previously tested.
          (zeertzjq)

closes: vim/vim#13985

https://github.com/vim/vim/commit/ae07ebc04b0726e12b1af39d52e01d86ae79ef0a

Problem:  Modifying a hidden buffer still interferes with prompt buffer
          mode changes.
Solution: Save and restore b_prompt_insert.
          (zeertzjq)

closes: vim/vim#13875

Modifying hidden buffer still interferes with prompt buffer mode changes

https://github.com/vim/vim/commit/f267847017976ab85117bdf75b45e769836f8d69

Problem:  Insert mode not stopped if an autocommand modifies a hidden
          buffer while closing a prompt buffer.
Solution: Don't set b_prompt_insert if stop_insert_mode is already set.
          (zeertzjq)

closes: vim/vim#13872

https://github.com/vim/vim/commit/96958366ad6159efe708b694055320ed19357e61

- remove helpers.cur*meths
- remove helpers.nvim