After cbc8d72fde when editing
the command in the command editing window (q:, q/, q?) it was possible
to switch to the previous tab. Doing so put Nvim in a bad state.
Moreover, switching tabs via the other available mechanisms (gt, gT,
<C-W>gt, <C-W>gT) is not possible when in the command editing window.
Here, the behavior is prevented. It is no longer possible to switch to
the previous tab when editing the command in the command editing window.
The solution is to share code between gt, gT, and g<Tab>. Specifically,
goto_tabpage_lastused now calls through goto_tabpage rather than
directly calling goto_tabpage_tp. Doing so works well because all the
validation enjoyed by gt and gT is present in goto_tabpage.
29b1a4761a