GTK+ has two directories for translations: the default 'po' and the
additional 'po-properties' for the GObject properties translation
domain.

Since the content of the translations directories are filled by
autoreconf and gettext, and are duplicated between the two gettext
domains we use, we should handle this ad hoc inside git.mk, instead of
trying to catch up by adding po-properties files in GITIGNOREFILES.