:Gcommit -v opens message in new tab

Closes #513.  References #480.

plugin/fugitive.vim

@@ -1015,6 +1015,8 @@ function! s:Commit(args) abort
         endif
         if bufname('%') == '' && line('$') == 1 && getline(1) == '' && !&mod
           execute 'keepalt edit '.s:fnameescape(msgfile)
+        elseif a:args =~# '\%(^\| \)-\%(-verbose\|\w*v\)\>'
+          execute 'keepalt tabedit '.s:fnameescape(msgfile)
         elseif s:buffer().type() ==# 'index'
           execute 'keepalt edit '.s:fnameescape(msgfile)
           execute (search('^#','n')+1).'wincmd+'