Hints for gitg in main

Warnings

• ancient-metadata
  The AppStream metadata should be updated to follow a more recent version of the specification.
  Please consult the XML quickstart guide for more information.
• gettext-data-not-found
  Unable to find any translation data for specified Gettext domain 'gitg'.
• no-translation-statistics
  We could not extract any translation statitics, althout metadata hints were given that the data exists. Please check the <translation/> metainfo tag for spelling mistakes. It also makes sense to ensure all locale files are placed in the right directories (e.g. gettext .mo files in /usr/share/locale/*/LC_MESSAGES/