git configuration option names should be case insensitive

[talshorer]

tals $ git config foo.BarBaz 2
tals $ git config foo.BarBaz
2
tals $ git config foo.BarBaz 2
tals $ git config -l | grep -i foo.BarBaz
foo.barbaz=2
tals $ 

git treats its configuration option names as case-insensitive. This would be a no-brainer by using configparser (which GitPython does use), but for some reason the optionxform method was overriden to change this behaviour.