convert: subversion should use util.quotecommand to wrap args to popen2

All other callers of util.popen2 and util.popen3 do this, as well as direct
callers of subprocess.Popen.

hgext/convert/subversion.py

@@ -914,7 +914,7 b' class svn_source(converter_source):'
         arg = encodeargs(args)
         hgexe = util.hgexecutable()
         cmd = '%s debugsvnlog' % util.shellquote(hgexe)
-        stdin, stdout = util.popen2(cmd)
+        stdin, stdout = util.popen2(util.quotecommand(cmd))
         stdin.write(arg)
         try:
             stdin.close()