Do not show empty split for Git help <cmd> on Windows

Git for Windows shows help for its subcommands in a browser; so
`Git help rebase`, for example, brings up empty split.

This commit covers notable exceptions to that: `git help`,
`git help -a`, `git help -g`, `git help --man` and `git --help`.

Author: andrey-starodubtsev

autoload/fugitive.vim

@@ -3761,8 +3761,12 @@ function! fugitive#PagerFor(argv, ...) abort
   let args = a:argv
   if empty(args)
     return 0
-  elseif (args[0] ==# 'help' || get(args, 1, '') ==# '--help') && !s:HasOpt(args, '--web')
+  elseif len(args) == 1 && args[0] =~# '\v^(help|-h)$'
+    return 1
+  elseif len(args) == 2 && args[0] ==# 'help' && args[1] =~# '\v^(-[ag]|--man)$'
     return 1
+  elseif (args[0] ==# 'help' || get(args, 1, '') ==# '--help') && !s:HasOpt(args, '--web')
+    return has('win32') ? 0 : 1
   endif
   if args[0] ==# 'config' && (s:HasOpt(args, '-e', '--edit') ||
         \   !s:HasOpt(args, '--list', '--get-all', '--get-regexp', '--get-urlmatch')) ||