Properly decode post-commit svn diff contents to Unicode.

`SCMTool.get_change` is expected to return a Commit object with a 'diff'
parameter that is a unicode object. The SVN libraries both return diffs in
string format, which need to be decoded.

Testing done: Ran unit tests.

Fixes bug 3353.

Reviewed at https://reviews.reviewboard.org/r/5845/