rebase --interactive: Compute upstream SHA1 before switching branches

rebase --interactive: Compute upstream SHA1 before switching branches

If the upstream argument to rebase (the first argument) was relative to
HEAD and the name of the branch to rebase (the second argument) was given,
the upstream would have been interpreted relative to the second argument.
In particular, this command

    git rebase -i HEAD topic

would always finish with "Nothing to do". (a1bf91e fixed the same issue
for non-interactive rebase.)

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>