If working with git-svn, the code must set merge_base as the branch that references the remote svn trunk. Not completely sure if master should be hardcoded in here.
First the parent diff must be calculated, that will fill up the _git_svn_rev_memory attr and that will allow diff_line work fine later.
Fix git-svn uncommited parent branch errors
Review Request #2496 — Created July 24, 2011 and discarded
Working with multiple branches allow posting reviews using --parent option over uncommited branches. With the following branch structure o master \ o---o topicA \ o---o topicB posting a review from topicB (using --parent=topicA) will show changes from boths branches but will only highlight topicB ones.