Extend lazy detection to netrw

The requires dropping support for b:netrw_curdir, which was added in
fecd42864a for the vague reason of
handling "unusually named netrw buffers".  I'll figure out what to do
about those alleged buffers when one of them surfaces again.

plugin/fugitive.vim

@@ -616,9 +616,8 @@ let g:io_fugitive = {
 augroup fugitive
   autocmd!
 
-  autocmd BufNewFile,BufReadPost *
-        \    call FugitiveDetect(+expand('<abuf>'), 0)
-  autocmd FileType           netrw call FugitiveDetect(get(b:, 'netrw_curdir', +expand('<abuf>')), 1)
+  autocmd BufNewFile,BufReadPost *  call FugitiveDetect(+expand('<abuf>'), 0)
+  autocmd FileType           netrw  call FugitiveDetect(+expand('<abuf>'), 0)
 
   autocmd FileType git
         \ call fugitive#MapCfile()