Problem:    Various code not covered by tests.
Solution:   Add a few more test. (Yegappan Lakshmanan, closes vim/vim#7995)
https://github.com/vim/vim/commit/1f448d906b3c516e5864dc5bae3ddbf3664ee649

Cherry-pick some test_edit.vim changes from patches 8.2.{1022,1432}.
Reorder test_undo.vim to match upstream.

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:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes vim/vim#5645)
https://github.com/vim/vim/commit/f0cee1971f5258ce61f8a4e6a04d35c1e625bb01

Cherry-pick Test_bunload_with_offset() from patch 8.2.0243

Problem:    No test for what 8.1.0052 fixes.
Solution:   Add a test. (closes vim/vim#10531)
https://github.com/vim/vim/commit/3760bfddc414e4d3e1c4203db8c22e293cf08d09

Problem:    maparg() may return a string that cannot be reused.
Solution:   use msg_outtrans_special() instead of str2special().
            (closes vim/vim#10384)
https://github.com/vim/vim/commit/0519ce00394474055bd58c089ea90a19986443eb

Problem:    Listing of mapping with K_SPECIAL is wrong.
Solution:   Adjust escaping of special characters. (closes vim/vim#10351)
https://github.com/vim/vim/commit/ac402f4d64bec6b6efd809fef52f5b34627bf947

Avoid overshadowing.
Cherry-pick Test_list_mapping() from Vim patches 8.2.{0148,2994}.
Fix rhs_is_noop.

Problem:    K_SPECIAL may be escaped twice.
Solution:   Avoid double escaping. (closes vim/vim#10340)
https://github.com/vim/vim/commit/db08887f24d20be11d184ce321bc0890613e42bd

Problem:    Some mapping code is not fully tested.
Solution:   Add more test cases. (Yegappan Lakshmanan, closes vim/vim#5519)
https://github.com/vim/vim/commit/c2a60ae10e7badad9186de59a9994fc8c9f539e0

Problem:    Fix for unmapping simplified key not fully tested.
Solution:   Add a test case. (closes vim/vim#10292)
https://github.com/vim/vim/commit/abeb09b2c53054513812d1e56716e2a5abe8f354

Problem:    Unmapping simplified keys also deletes other mapping.
Solution:   Only unmap a mapping with m_simplified set. (closes vim/vim#10270)
https://github.com/vim/vim/commit/a4e3332650021921068ef12923b4501c5b9918cb