git-gui: various French translation fixes

Mostly grammar, spelling and typography fixes, but also a few wording
enhancements here and there.

Signed-off-by: Sam Hocevar <sam@zoy.org>
Acked-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>