git-gui: various French translation fixes

Mostly grammar, spelling and typography fixes, but also a few wording
enhancements here and there.
Signed-off-by: NSam Hocevar <sam@zoy.org>
Acked-by: NChristian Couder <chriscool@tuxfamily.org>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>