Support WithProperties in branch option of various source steps.

Fixes #752

Signed-off-by: Tom Prince <tom.prince@ualberta.net>

master/buildbot/steps/source.py

@@ -227,7 +227,7 @@ def start(self):
         # what source stamp would this build like to use?
         s = self.build.getSourceStamp()
         # if branch is None, then use the Step's "default" branch
-        branch = s.branch or self.branch
+        branch = s.branch or properties.render(self.branch)
         # if revision is None, use the latest sources (-rHEAD)
         revision = s.revision
         if not revision and not self.alwaysUseLatest: