Let read-tree update index and tree

Remove the unnecessary additional call to 'git checkout' to update the
working tree to the current index when finishing the import merge. Add
the necessary options to the read-tree call to update both.

Additionally add some basic tests to ensure no behavioural change.

Change-Id: I4bd77aaf0b6c9b71e717d74db92af0d897d9c7b3