don't replace existing build when running with prefer-new

Only try to replace a build with newer one when the required one is missing.
It also means the buildroot will be closer to the one in primary.

util/koji-shadow

@@ -730,7 +730,7 @@ class BuildTracker(object):
                 if depth > 0:
                     print "%sDep replaced: %s->%s" % (head, build.nvr, replace)
                     return build
-            if options.prefer_new and not options.build:
+            if options.prefer_new and not (options.build or build.state == "common"):
                 latestBuild = self.newerBuild(build, tag)
                 if latestBuild != None:
                     build.substitute = latestBuild.nvr