Problem:    Various code not covered by tests.
Solution:   Add more test coverage. (Yegappan Lakshmanan, closes vim/vim#5720)
https://github.com/vim/vim/commit/91ffc8a5f5c7b1c6979b3352a12ed779d11173a9

Test_Ex_echo_backslash() is not applicable to Vim enhanced Ex mode.

Problem:    Digraph test fails when LC_ALL is set to "C".
Solution:   When restoring 'encoding' set it to "utf-8". (closes vim/vim#8742)
https://github.com/vim/vim/commit/52eb372a04dfc5d5afef238c1b3c4a8e92020837

Problem:    New digraph functions use old naming scheme.
Solution:   Use the digraph_ prefix. (Hirohito Higashi, closes vim/vim#8580)
https://github.com/vim/vim/commit/29b857150c111a455f1a38a8f748243524f692e1

Problem:    Cannot add a digraph with a leading space.  It is not easy to list
            existing digraphs.
Solution:   Add setdigraph(), setdigraphlist(), getdigraph() and
            getdigraphlist(). (closes vim/vim#8580)
https://github.com/vim/vim/commit/6106504e9edc8500131f7a36e59bc146f90180fa

Use GA_APPEND_VIA_PTR in registerdigraph().
Use tv_list_append_*() in getdigraphlist_appendpair().
Put the error messages in digraph.c.
E196 is N/A.
Remove mentions about 'encoding' being non-Unicode.
Nvim doesn't support setting encoding=japan, so skip a test.

Problem:    Various typos in source files and tests.
Solution:   Fix the typos. (Emir Sari, closes vim/vim#5532)
https://github.com/vim/vim/commit/4b96df5a017a04141c4e901b1fc5704a3ca48099

Problem:    Some errors are not covered by tests.
Solution:   Add a few more tests. (Dominique Pelle, closes vim/vim#5920)
https://github.com/vim/vim/commit/067297e16a516838dbc46aaa9d8b1a507afec28d

Problem:    Tabpage menu and tabline not sufficiently tested.
Solution:   Add tests. (Yegappan Lakshmanan, closes vim/vim#6307)
https://github.com/vim/vim/commit/8c524f76ebd43f006e765534765b595de7095f12

Cherry-pick Test_entering_digraph() from patch v8.2.1022.
Cherry-pick :CheckGui from patch v8.1.1826.

Problem:    Too many #ifdefs.
Solution:   Graduate FEAT_MBYTE, the final chapter.
https://github.com/vim/vim/commit/30276f2beb248557c6b33cd5418bca8b7084b0a5

Problem:    :digraph! not tested.
Solution:   Add a test. (Dominique Pelle, closes vim/vim#5925)
https://github.com/vim/vim/commit/e3242346cf1c2ff8b0315cb2cb98e731bfa952c0

Problem:    Displayed digraph for "ga" wrong with 'encoding' "cp1251".
Solution:   Convert from 'encoding' to "utf-8" if needed. (closes vim/vim#3015)
https://github.com/vim/vim/commit/bc5020aa4d7ef4aea88395eff858f74fc881eab9