Fix remote updating

`git remote update` requires a named remote not a url. Previously jeepyb
provided a url which caused git to fail and jeepyb to not update ACLs.
Don't provide any remote names instead and fall back on the default
behavior to update everything.

Change-Id: I8e7113db5355e81faeff1f97b037bc41783b9a8c
diff --git a/jeepyb/cmd/manage_projects.py b/jeepyb/cmd/manage_projects.py
index 2c4eec7..9ebca86 100644
--- a/jeepyb/cmd/manage_projects.py
+++ b/jeepyb/cmd/manage_projects.py
@@ -136,7 +136,7 @@
     # one yet.
     output = ""
     for x in range(10):
-        status = git_command(repo_path, "remote update %s" % remote_url, env)
+        status = git_command(repo_path, "remote update --prune", env)
         if status != 0:
             log.error("Failed to update remote: %s" % remote_url)
             break