Better solution for doc/tags dirtying sub-modules

Previously I added "ignore = dirty" for each sub-module but this means
that sub-modules in a dirty state would also be ignored when things
other than doc/tags had dirtied them. Also it makes adding sub-modules
less fun because you have to add this line and re-stage the .gitmodules
file each time you add a sub-module.

I solved this by adding doc/tags to the global ignore file (I forgot I
had one) so now I can safely remove all these lines. As a last fix, the
sparkup plugin also had to be updated because git was actually tracking
the doc/tags file in that sub-module.