Set the 'revision' property more often

If the requested SourceStamp was HEAD, and we calculate a revision from
the set of Changes passed in, then set the 'revision' property
accordingly.  Fixes buildbot#101.

buildbot/steps/source.py

@@ -206,6 +206,10 @@ def start(self):
         revision = s.revision
         if not revision and not self.alwaysUseLatest:
             revision = self.computeSourceRevision(s.changes)
+            # the revision property is currently None, so set it to something
+            # more interesting
+            self.setProperty('revision', str(revision), "Source")
+
         # if patch is None, then do not patch the tree after checkout
 
         # 'patch' is None or a tuple of (patchlevel, diff, root)