gdal_translate: don't recopy band units if rescaling or unscaling is involved (mentionned in comment 14 of #3085)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27928 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx