git-gui: Initial french translation

Here are some of the choices made to translate Git Gui to french:

- commit         -> "commit" (noun) or "commiter" (verb)
- stage (index)  -> "pré-commit" (noun) or "pré-commiter" (verb)
- (re)scan       -> "(re)synchroniser"
- reset          -> "réinitialiser"
- checkout       -> "emprunt" (noun) or "emprunter" (verb)
- revision expression -> "expression de révison"

I am not completely happy with these, but it's a start...

[sp: Inserted a missing LF in message on line 466]

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>