git-svn revision support

I do a lot of reviewing of code hosted in an SVN repo; developers will often reference the SVN revision number of changes. This patch adds another column to the history view that shows the SVN revision number, extracted from the `git-svn-id` line. This allows for quickly mapping between git and SVN commits.

In quick testing, performance doesn't seem to be heavily impacted, but if others find differently it could be an off-by-default option.

(Apologies also for the XIB diff spam, I'm never sure which of the bits IB changes are actually required, so I played it safe and committed the whole thing)