My point was that the dev knows "sta" is currently available as an abbreviation for "start", and when he adds "status" it no longer will be. So if he wants to maintain strict backwards compatibility he should add it back in manually. It's no longer unambiguous, but it does maintain the previous behavior.
Of course I personally would go with option two, where its documented that the abbreviations exist but are subject to change and you shouldn't rely on them.
The dev also "knows" that 'star' is available as an abbreviation. And in a realistic set of options for a real tool like git, there might be combinatorially many possible abbreviations which he now has to "know" and think about every possible intersection or ambiguity.
Of course I personally would go with option two, where its documented that the abbreviations exist but are subject to change and you shouldn't rely on them.