Prune remote branches when mirroring.

* jeepyb/cmd/run_mirror.py: Use git fetch -p origin so deleted
remote branches will be removed locally before processing. Also deal
with pip freeze warnings which might include "==" by skipping lines
containing spaces when looking for package versions to download, and
make sure to match "-e" lines as well.

Change-Id: I646ce937b59da211d03ae2d3b09c9aabd10cb91e
Reviewed-on: https://review.openstack.org/23494
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins