DRY up externalize-project.sh

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
externalize-project: skip trailing underscore in repository URL

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
TopoJ_ is an external project now

As of 15840de(Add a .gitignore file), TopoJ_ lives in

	https://github.com/fiji/TopoJ

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ignore if TopoJ_ is cloned into external/

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Adjust .gitignore

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix the TopoJ_.version definition

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
externalize-project: handle absence of <version> gracefully

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>