Fix .po data merging in l10n_completion.py
Fix .po data merging in l10n_completion.py

This applies the changes I just made to the kitsune version so that it's
correctly merging numbers between .po files in a given locale.

It also provides locale-level percent completed numbers.

DESHRAJ/fjord,hoosteeno/fjord,mozilla/fjord,rlr/fjord,rlr/fjord,lgp171188/fjord,hoosteeno/fjord,rlr/fjord,hoosteeno/fjord,hoosteeno/fjord,Ritsyy/fjord,mozilla/fjord,DESHRAJ/fjord,Ritsyy/fjord,staranjeet/fjord,lgp171188/fjord,mozilla/fjord,Ritsyy/fjord,DESHRAJ/fjord,staranjeet/fjord,lgp171188/fjord,staranjeet/fjord,rlr/fjord,lgp171188/fjord,staranjeet/fjord,mozilla/fjord,Ritsyy/fjord