The highlight group is
hi gitcommitOverflow. however it's part of vim's built-in
gitcommit.vim, not in any way part of fugitive (besides having the same maintainer).
Strangely, it looks like it was intended to be linked to
Error by default but this was commented out.
"hi def link gitcommitOverflow Error
Putting this line in your custom color scheme/syntax configuration would work, or you can assign different colors in the usual way.
In the future, if you want to see which highlight group is being used you can use a map such as this (from vim tip 99 and How to identify the highlight groups in use on a particular character?):
map <F10> :echo "hi<" . synIDattr(synID(line("."),col("."),1),"name") . '> trans<'
\ . synIDattr(synID(line("."),col("."),0),"name") . "> lo<"
\ . synIDattr(synIDtrans(synID(line("."),col("."),1)),"name") . ">"<CR>