i18n: merge i18n comments of translatable texts correctly Before this patch, i18n comments of translatable texts are lost at creation of hg.pot file, if: - same translatable text appears multiple times, - the 1st appearance does not have i18n comment, and - any of rest has it For example, previous patch for filemerge.py adds translatable texts with i18n comments, but these comments are lost, because: - automatically added texts in docstring of internal merge tools are picked up earlier than these translatable texts, because of location in filemerge.py - but docstring has no i18n comment This patch makes addentry() of posplit merge i18n comments of later translatable texts, in order to keep them at creation of hg.pot.