* app/core/gimpdata.c (gimp_data_get_identifier): check if the data's
  path starts with either of those and use the symbolic paths in that
  case.

* data/tags/gimp-tags-default.xml.in: use them here too instead of
  /home/martin/foo/bar/...

This way we can ship a default file that makes sense, and need much
less identifier remapping. The identifiers even stay the same when
upgrading GIMP.