There were a few bugs making it impossible to use --parent with git-svn and

git-perforce checkouts. The first of which is that the make_*_diff methods

needed to have access to the merge base, rather than the base of the individual

diff. I've plumbed this through so that we can properly determine the upstream

revision to include in the diff headers.
The other problem was that the git-svn code was mistakenly using --parent to

mean the upstream branch rather than --tracking-branch.

Posted changes with a git-svn checkout, both for a single branch off of master,

as well as using --parent to post a change with un-committed parent revisions.