automatically disable ^G modifier after successful auto-completions

This will be important once we're doing LSP auto-completions.
Until a completion is complete, we do NOT want to disable the modifier however
since otherwise it can be disabled after a partial completion if there happened
to be a rubbed out command line.
Discarding a rubbed out command line consequently no longer disables
the modifier as well.

Any change of the modifier is now logged with an Info-level message.

Some rudimentary test case has been added for rub-in via ^G.

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