gitserver: Store repository size in gitserver_repos table

Created by: asdine

We currently don't have any information about the disk space taken by each repository. Having this information in the database is a pre-requisite for RFC 595 so that we can quickly calculate if shards have enough disk space before running the migration. The Cloud and Search teams have also expressed interest in this information.

Add a new column to the gitserver_repos table and, every time a repo is cloned or updated, we must update that column with the size of the repository directory on disk. Note that repos only contain .git directories.