The reason for this was because an inexperienced user might
mistakenly think what is shown with the last-minute diff
contains all the change that is being committed (instead, what
the user asked to check was an incremental diff since what has
been staged so far). However, this turns out to only annoy
people who know what they are doing. Inexperienced people
would not be using the first "update-index" anyway, in which
case they would see the full changes in the "git diff".