git-gui: use textconv filter for diff and blame

git-gui: use textconv filter for diff and blame

Create a checkbox "Use Textconv For Diffs and Blame" in git-gui options.
If checked and if the driver for the concerned file exists, git-gui calls diff
and blame with --textconv option

Signed-off-by: Clément Poulain <clement.poulain@ensimag.imag.fr>
Signed-off-by: Diane Gasselin <diane.gasselin@ensimag.imag.fr>
Signed-off-by: Axel Bonnet <axel.bonnet@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>