The command line tool msgmerge is used to merge two PO files into one. It is really easy to use, here is an example:

msgmerge updated-po-file reference-file > output.po

The result in output.po contains the reference-file with the new translated strings from the updated-po-file.

This tool can be used to merge strings from the development branch towards a stable branch or vice-versa.

Example with a PO file:

msgmerge ~/Downloads/fr.po fr.po > fr.po.new
mv fr.po.new fr.po

Note that you cannot send the output directly on fr.po because it will fail with an 100% untranslated PO file.