Problem:    No error when using :complete for :command without -nargs.
Solution:   Give an error. (Martin Tournoij, closes vim/vim#8544, closes vim/vim#8541)
https://github.com/vim/vim/commit/de69a7353e9bec552e15dbe3706a9f4e88080fce

N/A patches for version.c:

vim-patch:8.1.1801: cannot build without the +eval feature

Problem:    Cannot build without the +eval feature.
Solution:   Always define funcexe_T.
https://github.com/vim/vim/commit/505e43a20eb25674b18d73971fe3b51dad917f9a

vim-patch:8.1.1818: unused variable

Problem:    Unused variable.
Solution:   Remove the variable. (Mike Williams)
https://github.com/vim/vim/commit/b4a88a0441a65a0c9411c294825a08ca703f541e

vim-patch:8.2.1464: Vim9: build warning for unused variable

Problem:    Vim9: build warning for unused variable.
Solution:   Delete the variable declaration.
https://github.com/vim/vim/commit/829ac868b7615d73dbfb536f7fcd44fc7c5b7c1d

vim-patch:8.2.2639: build failure when fsync() is not available

Problem:    Build failure when fsync() is not available.
Solution:   Add #ifdef.
https://github.com/vim/vim/commit/5ea79a2599d35f75e1ae8a75d2711c754c4cb7c4

vim-patch:8.2.2814: Vim9: unused variable

Problem:    Vim9: unused variable. (John Marriott)
Solution:   Adjust #ifdef.
https://github.com/vim/vim/commit/b06b50dfa06e1cbefd634e2735e7cd5ddd5b911c

vim-patch:8.2.2947: build failure without the channel feature

Problem:    Build failure without the channel feature.
Solution:   Add back #ifdef. (John Marriott)
https://github.com/vim/vim/commit/f5bfa8faa7bbe025c10148d37e8b47217a430a3b

vim-patch:8.2.2976: build failure without the +eval feature

Problem:    Build failure without the +eval feature.
Solution:   Add #ifdefs.
https://github.com/vim/vim/commit/8de901e1f1b051e02a61ae76ad7c925e4c0642e5

vim-patch:8.2.2986: build failure without the profile feature

Problem:    Build failure without the profile feature.
Solution:   Add #ifdef.
https://github.com/vim/vim/commit/d9f31c13d217b4b97f724774a67a6d1f8640e8ae

vim-patch:8.2.3114: Amiga-like systems: build error using stat()

Problem:    Amiga-like systems: build error using stat().
Solution:   Only build swapfile_process_running() on systems where it is
            actually used. (Ola Söder, closes vim/vim#8519)
https://github.com/vim/vim/commit/599a6e5b3629d943a795cd69e4d3d19886f86405

Eliminate `expect_err` in favor of `pcall_err` + `eq` or `matches`.

builtin commands are never buffer-local, so we can return empty for that
case.

Support more :command attributes:
  -bang
  -bar
  -register

- Always return all keys, with at least NIL value.
- Require `opts` param to be {"builtin":false}
- Validate `opts` param