Use fetch, not pull, to update bare repos

Pull expects a working directory (because it does a fetch, then a merge), so we should actually just use fetch. Specifying `origin master:master` also ensures our local master’s HEAD is updated to match origin master, and `--tags` ensures (hopefully) that we update all the local tags, too.

Author: SteveMarshall

templates/default/source-mirror.sh.erb

@@ -6,7 +6,7 @@
 local_dir=<%= node.source_mirror.data_dir %>/<%= repo_owner %>/<%= repo %>.git
 if [ -d $local_dir ] ; then
   cd $local_dir
-  git pull
+  git fetch --tags origin master:master
   cd -
 else
   mkdir -p `dirname $local_dir`
@@ -17,4 +17,4 @@ else
   cd -
 fi
 <% end %>
-<% end %>
+<% end %>