Short circuit when jumping too far in blame history

References https://github.com/tpope/vim-fugitive/issues/607

plugin/fugitive.vim

@@ -2070,6 +2070,7 @@ function! s:BlameJump(suffix) abort
   if winnr > 0
     exe bufnr.'bdelete'
   endif
+  if exists(':Gblame')
     execute 'Gblame '.args
     execute lnum
     let delta = line('.') - line('w0') - offset
@@ -2079,6 +2080,7 @@ function! s:BlameJump(suffix) abort
       execute 'normal! '.(-delta)."\<C-Y>"
     endif
     syncbind
+  endif
   return ''
 endfunction