Refactor SVN's filter diff into a utility function

Review Request #6357 — Created Sept. 22, 2014 and submitted — Latest diff uploaded

Information

RBTools
release-0.6.x
b188fca...

Reviewers

Move SVNClient._filter_diff into rbtools.utils.diffs so that it may be
used by other SCMClient instances that require filtering.

Running rbt diff -X file with multiple files modified yields the diff
for all files modified except file.