I know. I don’t understand why the Linux Kernel uses a hard-coded abbreviation instead of just letting Git figure out how long it should be (edited my comment now).
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.