git: only list heads and tags references

When looking for git source references, we only need the heads.
This reduce the load of using https://opendev.org/zuul/zuul-jobs
which now contains all the gerrit refs.

Change-Id: I72532592312062dee17f095f3d56866d16e429dc

zuul/driver/git/gitconnection.py

@@ -184,6 +184,7 @@ class GitConnection(BaseConnection):
         refs = {}
         client = git.cmd.Git()
         output = client.ls_remote(
+            "--heads", "--tags",
             os.path.join(self.baseurl, project))
         for line in output.splitlines():
             sha, ref = line.split('\t')