Git hasn't used "seven-character prefixes when there are no collisions" in a long time.
It's a combination of the "repo size" (as in, estimated number of objects) and a hard floor of seven characters.
You can see this by running "git log --oneline=7" on any non-trivially sized repository (e.g. linux.git). There's plenty of hashes that uniquely abbreviate to 7 characters, but they're currently all shown with 12 by default.
There may be some extra trigger that causes it to go beyond seven for everything, I don’t know (never worked on a repository anywhere near that large), but there’s certainly still at least some form of collision logic in there (and this is why I said what I said, because I’ve used lucky_commit enough to experience it):
$ git init x
Initialized empty Git repository in /tmp/x/.git/
$ cd x
$ git commit --allow-empty -m one
[master (root-commit) 4144321] one
$ git log --oneline
4144321 (HEAD -> master) one
$ lucky_commit
$ git log --oneline
0000000 (HEAD -> master) one
$ git commit --amend --no-edit --reset-author --allow-empty
[master 3430e13] one
$ git log --oneline
3430e13 (HEAD -> master) one
$ lucky_commit
$ git log --oneline
0000000f (HEAD -> master) one
$ git reflog --oneline
0000000f (HEAD -> master) HEAD@{0}: amend with lucky_commit
3430e13 HEAD@{1}: commit (amend): one
00000005 HEAD@{2}: amend with lucky_commit
4144321 HEAD@{3}: commit (initial): one
$ git reflog expire --expire=now --all
$ git reflog --oneline
$ git log --oneline
0000000f (HEAD -> master) one
$ git gc --aggressive --prune=now
Enumerating objects: 2, done.
Counting objects: 100% (2/2), done.
Writing objects: 100% (2/2), done.
Total 2 (delta 0), reused 0 (delta 0), pack-reused 0
$ git log --oneline
0000000 (HEAD -> master) one
Yes, there's also a collision check, but it's not truncating to 7 characters and adding as needed to get past collisions. Rather it's truncating to N and then adding as needed. N=7 for a new repository, but it'll relatively quickly bump that to 8, then 9 etc
You don't need a very large repository to start bumping it up to 8 etc. E.g. my local redis.git is 9, some local few-thousand commit (partially automated) that I've only ever added to are at 8 etc.
This changed in v2.11 released in late 2016[1], but because the observable default on a new repository is 7 the "it's 7 unless collisions" has persisted in various places online.
All of which is to say that if you brute-force the first commit to be 0000001..., it'll start being displayed as <that><x>, where <x> is a random 0..9a..f character, unless you brute force to 8, 9 etc.
It's a combination of the "repo size" (as in, estimated number of objects) and a hard floor of seven characters.
You can see this by running "git log --oneline=7" on any non-trivially sized repository (e.g. linux.git). There's plenty of hashes that uniquely abbreviate to 7 characters, but they're currently all shown with 12 by default.