gitmodules: Set foldmethod=expr per modeline

Fold away every submodule entry.

Add section headers.

.config/vim/ftplugin/gitconfig.vim

@@ -0,0 +1,3 @@
+" Create folds for all lines between a section start and either the next section
+" start or blank line
+setlocal foldexpr=getline(v:lnum)=~'^\\['?'>1':getline(v:lnum+1)=~'(^\\[\\|^$)'?'<1':getline(v:lnum)=~'^\\t'