Write ref list when taking backups

gitlab#35862 (closed)

This is the beginning of the infrastructure we need to do incremental backups. Write refs so that we can calculate patterns to give to git bundle create to only backup new objects and switching to use CreateBundleFromRefList instead of assuming we'll backup all the objects all the time.

We want to start generating ref files now, so that if/when we start doing incremental backups most installations will already have refs available.

Ideally we would stop overwriting the old bundles/refs too but we can't until the old ruby implementation is deleted. Plus once we start restoring incremental backups we will need to be able to get a listing from the sink which hasn't been implemented yet.

Some small testing with the gitlab repo shows no significant change in performance.

Before:

$ time ./_build/bin/gitaly-backup create -path=/home/james/Downloads/backup < ~/Downloads/single.json 
INFO[0000] started backup                                gl_project_path=root/gitlab relative_path=@hashed/f5/ca/f5ca38f748a1d6eaf726b8a42fb575c3c71f1864a8143301782de13da2d9202b.git storage_name=default
INFO[0031] completed backup                              gl_project_path=root/gitlab relative_path=@hashed/f5/ca/f5ca38f748a1d6eaf726b8a42fb575c3c71f1864a8143301782de13da2d9202b.git storage_name=default

real	0m31.965s
user	0m7.051s
sys	0m4.912s

After:

$ time ./_build/bin/gitaly-backup create -path=/home/james/Downloads/backup < ~/Downloads/single.json 
INFO[0000] started backup                                gl_project_path=root/gitlab relative_path=@hashed/f5/ca/f5ca38f748a1d6eaf726b8a42fb575c3c71f1864a8143301782de13da2d9202b.git storage_name=default
INFO[0030] completed backup                              gl_project_path=root/gitlab relative_path=@hashed/f5/ca/f5ca38f748a1d6eaf726b8a42fb575c3c71f1864a8143301782de13da2d9202b.git storage_name=default

real	0m30.287s
user	0m7.576s
sys	0m4.240s