Use fetch instead of update in merger

According to a well written and therefore almost certainly correct
answer on SO[1], fetch and update are nearly the same, especially
when used in the context of a single remote (as we are here).

Since we sometimes want correct tag information as well as correct
branch information for our repos, switch from update to fetch, with
the '--tags' argument.

[1] http://stackoverflow.com/questions/2688251/what-is-the-difference-between-git-fetch-origin-and-git-remote-update-origin

Change-Id: I3a31b84e7d65efa5430fb29a77047ae4b16ad18c
diff --git a/zuul/merger/merger.py b/zuul/merger/merger.py
index 3bc29e6..94933de 100644
--- a/zuul/merger/merger.py
+++ b/zuul/merger/merger.py
@@ -182,7 +182,7 @@
         repo = self.createRepoObject()
         self.log.debug("Updating repository %s" % self.local_path)
         origin = repo.remotes.origin
-        origin.update()
+        origin.fetch(tags=True)
 
 
 class Merger(object):