Add git filter integration

[vidartf]

• Adds capability for applying git filters according to git config.
• Applies filter on remote as it should for calls to nbdiff with git refs.
• As diff drivers/tools cannot correctly apply filters, it will always apply any filter on remote if the flag --use-filter is given, otherwise it will not consider filters.

[vidartf]

This could still benefit from tests on nbdiff with gitrefs, and on the git difftool.