When not using interactive mode, ensure that git rebase is called as a
subprocess to ensure git-upstream can switch the final branch to the
merged result after completion of the import.
Only where interactive mode is enabled should the branch remain on the
import branch after running to completion and leave it to the user to
switch to the target branch, as otherwise git-rebase will throw an
error.
Change-Id: I633a371775b5a177c76529a49b44fb841d9c1332
Related-Bug: #1625876
a24debcbf8