Alt solution : https://gist.github.com/hieblmedia/9318457

Must look if we can do :
!/plugins/
/plugins/**
!/plugins/Demo/
!/plugins/AuditLog/
!/plugins/index.html

And after add a new 'limesurvey distributed plugin'.

Maybe in 2 step
1st commit
!/plugins/
/plugins/**
!/plugins/Demo/
!/plugins/AuditLog/
!/plugins/NewPlugin/
!/plugins/index.html

2nd commit : add /plugins/NewPlugin/

I give a try

OK :

1. https://github.com/Shnoulle/LimeSurvey-Shnoulle/commit/e05dac14a5c9c309cc357e3112eac88eee60eba5
2. Add a plugin directory b: nothing happen
3. update .gitconfig : https://github.com/Shnoulle/LimeSurvey-Shnoulle/commit/2ddee13de890064109fb897490cca962804c2227