l10n: fr.po: Minor improvements

* Fix an occurrence of "dpuis" to "depuis".
* Add some entries in the translation index at the beginning of the
  file.
* Harmonize the spelling of various items based on how common each
  spelling or translation is throughout the file.
  * superproject -> super-projet
  * patch -> rustine
  * regex / regexp -> regex
  * regular expression -> expression régulière
  * loose object -> objet esseulé
  * directory -> répertoire
* Fix various typos (e.g.: trailing ".<" or ".", "mêm" -> "même")
* Fix minor grammatical errors (e.g: "le valeur" -> "la valeur")
* Remove old translations

Signed-off-by: Florian Sabourin <ethiraric@gmail.com>