gitk: Add French translation

Signed-off-by: NEmmanuel Trillaud <etrillaud@gmail.com>
Signed-off-by: NThomas Moulard <thomas.moulard@gmail.com>
Signed-off-by: NGuy Brand <gb@unistra.fr>
Signed-off-by: NNicolas Sebrecht <nicolas.s.dev@gmx.fr>
Signed-off-by: NPaul Mackerras <paulus@samba.org>