mergetools: vimdiff: fix single window layouts

mergetools: vimdiff: fix single window layouts

Layouts with a single window other than "MERGED" do not work (e.g.
"LOCAL" or "MERGED+LOCAL").

This is because as the documentation of bufdo says:

    The last buffer (or where an error occurred) becomes the current
    buffer.

And we do always do bufdo the end.

Additionally, we do it only once, when it should be per tab.

Fix this by doing it once per tab right after it's created and before
any buffer is switched.

Cc: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Reviewed-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>