externalize-project: edit .gitignore automatically

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

As of 4e70d0b(Add a .gitignore file), Trainable_Segmentation lives in

	https://github.com/fiji/Trainable_Segmentation

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

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

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Whoops. Use the correct quotes

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