This is caused by the following line from $VIMRUNTIME/syntax/gitcommit.vim
:
syn match gitcommitSummary "^.\" contained containedin=gitcommitFirstLine nextgroup=gitcommitOverflow contains=@Spell
You could just copy that syntax script to ~/.vim/syntax/
and modify it, but that drags you into maintaining your clone. I prefer to selectively change that single syntax definition in ~/.vim/after/syntax/gitcommit.vim
:
syn clear gitcommitSummary syn match gitcommitSummary "^.\" contained containedin=gitcommitFirstLine nextgroup=gitcommitOverflow contains=@Spell
Alternatively / in addition, you could open an issue at the project and ask for a configurable threshold; the hard-coded limit certainly isn't nice.