As stated in the title, I just wonder why ? I get that’s enough to identify a commit on phabricator but is there not a risk than a conflict appear and a commit share their first 12 characters ?
The risk is tiny and the consequences of a collision are not very consequential. The risk of collision can be seen in this table in the row for 12 hex digits: https://en.wikipedia.org/wiki/Birthday_problem#Probability_table
There are currently a bit more than 100,000 commits in the blender git repository. So that chance that any two 12-hex-digit prefixes collide is less than 1 in a million.
And what is the consequence if a collision happens? We might be confused as to which commit a build is from. And the consequence of that? Not much. Maybe some confusion in the minds of a developer trying to debug, but they’d likely be able to figure it out by trying both.
Of course, you still might wonder why not simply use 40 – I guess that is your question. I don’t know. In practice, devs like to use shorter than 40 just for ease of typing them in, but that’s a pretty minor advantage.
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Thanks you very much for answering my question.
After testing, phabricator deal with the problem and give the list of ambiguous commit if there is collision in the first hex-digit, so no worries 
git log --oneline give a 11 char hash. And as mentioned here, it is not a problem, really.
I personally use 12 long chars (manually trimmed) every time I have to refer to a hash in Blender. Why do I do that? Not sure, if memory serves me well, I used to use 10 chars in the past and realized git log was using 11, so I decided to up my game 