externalize-projects: ignore submodules

PIV_analyser started out as a submodule in src-plugins/; filter-branch
has a problem with that (fatal: failed to unpack tree object
5590d5ef93af9e5b64564307b88bbdbb0672fa64:src-plugins/PIV_analyser).
Let's just ignore those submodules...

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

As of aa0b37e(Add a .gitignore file), PIV_analyser lives in

	https://github.com/fiji/PIV_analyser

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

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

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

Memo to Johannes: test your fixes before committing.

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