autocompletion: add support for multi-autocompletion

[xaf]

For instance:

• e.g. cd command exists, and cd blah exists too.
• doing after cd will only autocomplete to cd blah
• but if cd allows for auto-completion, it should give a chance to cd to autocomplete too, at least until it's unsure yet that cd blah will take over if we were pressing "enter" right now