Problem:    Expanding <sfile> works differently the second time.
Solution:   Keep the expanded name when redefining a function. (closes vim/vim#5425)
https://github.com/vim/vim/commit/b9adef79eca6f95bc7376ff3a6a383e436c5d6ea

Problem:    This doesn't give the right result: eval(string(v:true)). (Nikolay
            Pavlov)
Solution:   Make the string "v:true" instead of "true".
https://github.com/vim/vim/commit/f48aa160fdd7b8caa7678e1a2139244dd2bdc547

Problem:    Subtracting from number option fails when result is zero. (Ingo
            Karkat)
Solution:   Reset the string value when using the numeric value.
            (closes vim/vim#8351)
https://github.com/vim/vim/commit/a42e6e0082a6d564dbfa55317d4a698ac12ae898

Cherry-pick Test_compound_assignment_operators() changes from patch 8.2.1593

Problem:    Various Ex commands not sufficiently tested.
Solution:   Add more test cases. (Yegappan Lakshmanan, closes vim/vim#5673)
https://github.com/vim/vim/commit/818fc9ad143911b2faa0d7cee86724aa70a02080

Needs to assert E170 instead of E580 because patch 8.2.3486 has been
ported but patch 8.2.1183 hasn't.

Problem:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes vim/vim#5649)
https://github.com/vim/vim/commit/bc2b71d44a0b90b6aeb3534a76912fccbe5577df

Problem:    Insufficient code coverage for ex_docmd.c functions.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#5618)
https://github.com/vim/vim/commit/9f6277bdde97b7767ded43a0b5a2023eb601b3b7

Cherry-pick Test_window_only() from patch 8.2.0203.
Cherry-pick a memory leak fix from patch 8.2.0399.

Problem:    Inconsistently using GetVimCommand() and v:progpath. (Daniel
            Hahler)
Solution:   Use GetVimCommand(). (closes vim/vim#4806)
https://github.com/vim/vim/commit/93344c2d707d9953f351c944e6a237c9916f69a3

Cherry-pick a change to test_profile.vim from patch 8.1.1544.
Cherry-pick a change to test_vimscript.vim from patch 8.1.1826.

Some of the args are no-op in Nvim, and `-i NONE` and `--headless` are
already added by `GetVimCommand()`. I'll try to match the order of args
in upstream, substituting `--not-a-term` with `--headless`.

Problem:    Test files still use function!.
Solution:   Remove the exclamation mark.  Fix overwriting a function.
https://github.com/vim/vim/commit/1e1153600c0377472d62cc553173fe555ddcf5a7

Some of the changes were already applied previously.

Problem:    copy() and deepcopy() fail with special variables. (Nikolai
            Pavlov)
Solution:   Make it work.  Add a test.  Closes vim/vim#614.
https://github.com/vim/vim/commit/155500077c80cdb5d9c63996000c011b66a676bf

Code is N/A. This only ports the tests.
Comment out tests involving v:none as Nvim has removed it.

Problem:    free_tv() can't handle special variables. (Damien)
Solution:   Add the variable type.
https://github.com/vim/vim/commit/6650a694547eb744afa060ec62dd8270e99db9f2

Code is N/A. This only ports the tests.