Vim patches for Vim help files must use Vim tags as diff hunk header
to reliably detect if a hunk is N/A or not.
Git does not know which files are Vim help files
so that it can use custom "diff.<filetype>.xfuncname" regexp
to create the header.
Update Neovim's gitattributes to detect Vim help files
and then use it on vim-patch.sh to scan Vim patches.
Add non-function tags from ":h vim_diff"
to detect N/A Vim runtime patches.