Store revs in ascii strings, not unicode.

This is to avoid Twisted complaining about unicode arguments being
passed to reactor.spawnProcess.

Also: release notes

Conflicts:
	master/buildbot/changes/gitpoller.py

master/buildbot/changes/gitpoller.py

@@ -166,9 +166,10 @@ def poll(self):
         log.msg('gitpoller: processing changes from "%s"' % (self.repourl,))
         for branch in branches:
             try:
-                revs[branch] = rev = yield self._dovccmd(
+                rev = yield self._dovccmd(
                     'rev-parse', [self._trackerBranch(branch)], path=self.workdir)
-                yield self._process_changes(rev, branch)
+                revs[branch] = str(rev)
+                yield self._process_changes(revs[branch], branch)
             except Exception:
                 log.err(_why="trying to poll branch %s of %s"
                         % (branch, self.repourl))