Support supports_parent_diffs in SVNRepositoryInfo
Support supports_parent_diffs in SVNRepositoryInfo

A change a while back (ad7dfc5) removed the `supports_parent_diffs`
parameter from the `SVNRepositoryInfo` constructor, causing an
unexpected keyword argument error when using `rbt post` with git-svn.
This patch fixes that regression.

Testing Done:
Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/8662/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools