[merge-ins, bzr]: use po_merge

[Georges Racinet]

Bzr has a useful po_merge plugin that avoids in particular missing some conflicts in translation files
(bzr help po_merge)

After a bit of code reading, it seems that one can use the global .bazaar/locations.conf or the local .bzr/branch/branch.conf but it is in any case necessary to list all i18n directories, which is both cumbersome and not dynamic enough for a big addons directory such as openobject-addons.

The recipe could manage automatically branch.conf to list all i18n repositories. The recipe already manages branch.conf to allow for transparent switching of parent branch.

Side note: from the code it's obvious that the correct option is 'po_merge.po_dirs', instead of the doc's 'po_merge.pot_dirs'