Don't process quickfix-loaded files, closes #176

Certain quickfix-commands like vimgrep have to load all searched files
into Vim and will execute the BufReadPost autocmd for all of them. Since
Tagbar doesn't need to generate tags for those files pause processing
while the command is running.

There doesn't seem to be a better way to find out whether vimgrep is
running than to set a temporary variable with the QuickFixCmdPre/Post
autocmds, see also
https://groups.google.com/forum/#!topic/vim_use/sUj5MFGmwD8

autoload/tagbar.vim

@@ -976,6 +976,9 @@ function! s:CreateAutocommands() abort
         autocmd BufDelete,BufUnload,BufWipeout * call
                     \ s:known_files.rm(fnamemodify(expand('<afile>'), ':p'))
 
+        autocmd QuickFixCmdPre  * let s:tagbar_qf_active = 1
+        autocmd QuickFixCmdPost * unlet s:tagbar_qf_active
+
         autocmd VimEnter * call s:CorrectFocusOnStartup()
     augroup END
 
@@ -3185,6 +3188,12 @@ endfunction
 function! s:AutoUpdate(fname, force) abort
     call s:LogDebugMessage('AutoUpdate called [' . a:fname . ']')
 
+    " This file is being loaded due to a quickfix command like vimgrep, so
+    " don't process it
+    if exists('s:tagbar_qf_active')
+        return
+    endif
+
     " Get the filetype of the file we're about to process
     let bufnr = bufnr(a:fname)
     let ftype = getbufvar(bufnr, '&filetype')