Add function tabgar:Update() to manually trigger AutoUpdate() (#633)

Co-authored-by: David Hegland <david.hegland@broadcom.com>

autoload/tagbar.vim

@@ -3555,6 +3555,12 @@ endfunction
 
 " }}}2
 
+" tagbar#Update() {{{2
+" Trigger an AutoUpdate() of the currently opened file
+function! tagbar#Update() abort
+    call s:AutoUpdate(fnamemodify(expand('%'), ':p'), 0)
+endfunction
+
 " tagbar#toggle_pause() {{{2
 function! tagbar#toggle_pause() abort
     let s:paused = !s:paused