JOSM: allow plugins to be built from a Git mirror - see #josm9450

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@30161 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4
JOSM/mirrored_download: apply URL encoding hack which has been removed from core (see #josm8566)

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@30162 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4