Add the ability to ignore spaces modifications in git diffs

In gitk, you have the choice to ignore spaces modifications in commits.
This means that in the diff display lines that differs only for number of spaces and tabs are not showed as different, so that if I refactor indentation (e.g convert from tabs to spaces) I do not see the whole file as modified, but only lines modified in real code.
This would be wonderful to be seen, at least as an option, in your diff tool.