So as noted you can configure the length, but you can't actually adjust the probability used by the auto-length formula.

The formula calculates how many bits are needed to store the approximate number of objects, then uses twice that many bits, rounding up.

For example, at 16000 objects git is still using the minimum length of 7 characters. But at 17000 objects it now takes 15 bits to store the object count, so it wants 30 bits of hash, which means 8 characters.

It can be configured with `core.abbrev`.

https://git-scm.com/docs/git-config#Documentation/git-config...