Pre-check repositories for updates

[rolinh]

Create a new option to allow pre-checking if a repository has received new commits since it was last updated. Currently, all repositories are updated "blindly", which is usually fine because when updates are actually available a pre-check would means unnecessary network traffic.
However, we are now considering moving data over to HDFS so it means that tar archives of repositories will need to be moved from HDFS prior to the update and placed there afterward. In this case, we want to avoid moving data from and to HDFS when unnecessary, hence a pre-check for remote changes is required.