> How about case-insensitive filesystems? [...] Negligible.

I don't buy this. Finding collisions is pretty easy. In one repository I have these files:

  rank2.cpp
  rank4.cpp
  rankN.cpp

The last two have the same case-insensitive Base84 encoding:

  mz{QkARW;1cB
  mz{QkaRW;1cB