[PATCH] l10n: add framework for localizing the manpages

This is first attempt at starting the internationalisation of the git man pages. For now, only man pages are generated, only for git-add and only in french. The chosen tool for this is po4a (PO for anything) which can ingest asciidoc files and spit po files for each structural entity. Then from the translated po files, the translated asciidoc files are merged.

The main advantage of po files over plain translated files is the ability to track changes in the original version and the fact that maybe the translators of git can more easily pick up the task with tools they already know. Po4a is ready for the simple addition of new languages.

The problem with po4a is that we don't know before hand how many files are created (it depends on the percentage of translated strings). This leads to invoke make recursively to have the list of available files after running po4a to actually process the generated asciidoc files.