Fix unicode errors with utf-8 files that have svn:keywords set.

In the case where a file in SVN had both unicode characters in it and
svn:keywords, trying to fetch the files would cause UnicodeDecodeErrors when
processing the keywords. I've fixed things up to do all the regex work in bytes
mode.

Testing done:
Ran unit tests. Before the changes to svn/base.py, the new test failed with the
same error as in the bug reports we've received about this. After, the test
runs fine.

Fixes bug 3425.

Reviewed at https://reviews.reviewboard.org/r/6052/