This is just for the abbreviated hash which git uses only for specific cases like display in the git log and similar, where it is a user experience improvement and relatively safe.
By that logic, any length of abbreviation will eventually fail if stored for a long time, those abbreviations should be seen as temporary for user interaction only. For the "Fixes:" footer they should have gone with full hash, it's hidden away in the footer so it doesn't disrupt anyone. Aanyone interacting with it will double click to copy it regardless if it is 8 or 40 characters long. Abbreviating it simply adds no value.
At what probability? Can that probability be configured as a global Git option?
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.
https://git-scm.com/docs/git-config#Documentation/git-config...