Tab completion for simplification modes

[isovector]

Set a CmdComplete CommandOption, and then follow the docs:

The following example lists user names to a Finger command 
    :com -complete=custom,ListUsers -nargs=1 Finger !finger <args>
    :fun ListUsers(A,L,P)
    :    return system("cut -d: -f1 /etc/passwd")
    :endfun

The following example completes filenames from the directories specified in
the 'path' option: 
    :com -nargs=1 -bang -complete=customlist,EditFileComplete
            \ EditFile edit<bang> <args>
    :fun EditFileComplete(A,L,P)
    :    return split(globpath(&path, a:A), "\n")
    :endfun

[isovector]

Fixed in 20210c4