The '--finish' option to the import command has no need of the upstream
branch, so allow it to be skipped by simply using the import branch name
as the upstream branch.
This still ensures the correct previous merge base will be found, but
since we have no need to scan the original upstream branch, can just use
the import branch named by the user instead and finish the merge.
Change-Id: I58ef4d05c52ecc684bebf5c9fecd6926871d3746
a4342f4474