vim-patch:9.1.1186: filetype: help files in git repos are not detected

Problem:  filetype: help files in git repos are not detected
Solution: detect */doc/*.txt files as help if they end with a help
          modeline, even if 'modeline' is off

Here's how I checked that this would still detect vim's own help files
correctly:

$ find . -type f -path '*/doc/*.txt' \
> -exec awk '{ } ENDFILE { print FILENAME ":" $0; }' '{}' + |
> grep -v 'vim:.*\<\(ft\|filetype\)=help\>'
./src/libvterm/doc/seqs.txt: 23    DECSM 42         = DECNRCM, national/multinational character

closes: vim/vim#16817

16d6fff98e

Split the pattern into a Lua pattern for the first part and a Vim regex
pattern for the second part, so that if the first part doesn't match
there is no need to use the Vim regex.

Co-authored-by: David Mandelberg <david@mandelberg.org>

runtime/lua/vim/filetype.lua

@@ -2415,7 +2415,13 @@ local pattern = {
     ['/boot/grub/menu%.lst$'] = 'grub',
     -- gtkrc* and .gtkrc*
     ['^%.?gtkrc'] = starsetf('gtkrc'),
-    ['^${VIMRUNTIME}/doc/.*%.txt$'] = 'help',
+    ['/doc/.*%.txt$'] = function(_, bufnr)
+      local line = M._getline(bufnr, -1)
+      local ml = line:find('vim:')
+      if ml and M._matchregex(line:sub(ml), [[\<\(ft\|filetype\)=help\>]]) then
+        return 'help'
+      end
+    end,
     ['^hg%-editor%-.*%.txt$'] = 'hgcommit',
     ['%.html%.m4$'] = 'htmlm4',
     ['^JAM.*%.'] = starsetf('jam'),

test/old/testdir/test_filetype.vim

@@ -916,8 +916,6 @@ func s:GetFilenameChecks() abort
     \         '.zcompdump', '.zlogin', '.zlogout', '.zshenv', '.zshrc', '.zsh_history',
     \         '.zcompdump-file', '.zlog', '.zlog-file', '.zsh', '.zsh-file',
     \         'any/etc/zprofile', 'zlog', 'zlog-file', 'zsh', 'zsh-file'],
-    \
-    \ 'help': [$VIMRUNTIME .. '/doc/help.txt'],
     \ }
 endfunc
 
@@ -1620,6 +1618,23 @@ func Test_haredoc_file()
   filetype off
 endfunc
 
+func Test_help_file()
+  filetype on
+  call assert_true(mkdir('doc', 'pR'))
+
+  call writefile(['some text', 'vim:ft=help:'], 'doc/help.txt', 'D')
+  split doc/help.txt
+  call assert_equal('help', &filetype)
+  bwipe!
+
+  call writefile(['some text'], 'doc/nothelp.txt', 'D')
+  split doc/nothelp.txt
+  call assert_notequal('help', &filetype)
+  bwipe!
+
+  filetype off
+endfunc
+
 func Test_hook_file()
   filetype on