Git Gui: initial french translation: fr.po

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...

Signed-off-by: Christian Couder <redacted>
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I just attached the file instead of sending the patch because I fear
some encoding breakages.