I think everything has settled on /usr/share/metainfo by now. ristretto.git#master:Makefile.am still uses "appdatadir = $(datadir)/appdata" which should be replaced with "$(datadir)/metainfo". see bug#15341
Igor referenced this bugreport in commit 8f66ae3fd139729203a8901b2ba5a648470c5d7f Place appdata.xml to metainfo dir https://git.xfce.org/apps/ristretto/commit?id=8f66ae3fd139729203a8901b2ba5a648470c5d7f
Fixed, thanks!