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/
Fix the close-submitted hook docs in the 2.0 release notes.

These docs were incorrect, showing a URL that doesn't actually work.
It was missing the hosting service ID. I've updated the URL, added the
list of supported IDs, and genericified some text that was specifically
calling out GitHub/BitBucket.

Reviewed at https://reviews.reviewboard.org/r/5850/
Fix typos and a misleading reference to GitHub in the release notes.

This fixes a typo ("post-receive" instead of "post-recieve") and changes
a sentence that was referencing GitHub but should have been more
generic.