Allow <C-p> pass through to ctrlp.vim in :Gstatus

This only works on the first line.  Any more than that, and we'll have
to give up on the whole <C-p> map entirely (which is not off the table).

plugin/fugitive.vim

@@ -706,10 +706,14 @@ function! s:StageNext(count)
 endfunction
 
 function! s:StagePrevious(count)
+  if line('.') == 1 && exists(':CtrlP')
+    return 'CtrlP '.fnameescape(s:repo().tree())
+  else
     for i in range(a:count)
       call search('^#\t.*','Wbe')
     endfor
     return '.'
+  endif
 endfunction
 
 function! s:StageReloadSeek(target,lnum1,lnum2)