geany-plugins-0.21 doesn't seem to support Subversion 1.7. I think the problem is the .svn directory is only in the top directory of the working copy. Just commenting the test for .svn in in_vc_svn() seems to get this working for me (not a real solution I guess!).
Also needed for me with Subversion 1.7 the patch changes the startdir for SVN_CMD_DIFF_FILE to VC_COMMAND_STARTDIR_FILE.