build(vim-patch): detect N/A tags in vimhelp patches #40942

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.

scripts/vim-patch.sh

@@ -927,9 +927,11 @@ is_na_patch() {
   for file in $FILES_REMAINING; do
     case ${file} in
       runtime/doc/*.txt | runtime/pack/dist/opt/*/doc/*.txt)
-        HUNKS=$(git -C "${VIM_SOURCE_DIR}" diff-tree --no-commit-id -r -b -U0 \
+        HUNKS=$(git -c core.attributesfile="$NVIM_SOURCE_DIR"/.gitattributes -c 'diff.helphelp.xfuncname=^.*\*[.a-zA-Z0-9\-]+\*$' -C "${VIM_SOURCE_DIR}" \
+          diff-tree --no-commit-id -r -b -U0 \
           '-I\*\s+For Vim version [0-9]\.[0-9]\.\s+Last change: [0-9]+ [A-Z][a-z]+ [0-9]+' \
           '-I compiled \(with\|without\) .*(\|.*\|) feature\.$' \
+          '-I^=+$' \
           "$patch" -- "${file}" |
           grep -v -e '{.\+ \(available\|compiled\) \(with\|without\) .\+}' |
           grep -Pzo '(?<=\n)@@ -[0-9][^@\n]+\+[0-9][^@\n]* @@[^@\n]*\n(?=([-+][^\n]*\n)+(@|$))' |