Turns out git-svn support hasn't worked out of the box in many years,

which shows how rarely people are using it anymore. The work formerly

done to add smart remote and ancestor branch detection failed if

explicit parents and tracking branches weren't provided.
The reason is that a git-svn remote is fake. There's a reference, and

it's in the .git/config file, but it doesn't exist as a proper remote.

Because of this, all remote-related checks failed.
On top of that, diff building failed due to Python string issues and

just some bad logic that didn't catch many operations.
This change fixes all that. We now handle the fake git-svn remote when

attempting to determine remotes and tracking branches.
Our ancestor detection no longer breaks when the common ancestor is the

first commit in the repository (which can occur for new repositories

here).
Diff building now uses the proper string types, and brings some fixes

over from the recent git-p4 diff building fixes to ensure proper diffs

when there are spaces in filenames or files are deleted.

Renames/moves/copies don't work, but there's a lot wrong with this in

Subversion diffs anyway.

Unit tests pass.
Manually tested with a fresh new local git-svn clone. Verified I was

able to generate working diffs from added, deleted, and edited changes

to files.