New vim-job based CallAsync doesn't handle complex commands correctly

[ardagnir]

It looks like the new job version of CallAsync is breaking up commands incorrectly.

Example of broken behavior:
let g:command = maktaba#syscall#Create(['echo', 'a']).And(['echo','b'])

g:command.Call (and CallAsync with vim jobs disabled) gives the correct stdout of "a\nb"
with vim jobs, g:command.CallAsync gives the incorrect stdout of "a && echo b"