Incremental restores

gitlab#35862 (closed)

Allows restoring backups from a series of incremental bundles while maintaining compatibility with existing backups.

The filenames used by the pointer locator changed here, but it is so far unused so should not be considered a breaking change.

As a sanity check I've run some select backup tests on gitlab-rails and checked that rake gitlab:backup:restore works using this branch:

...
time="2021-09-29T14:45:54+13:00" level=info msg="completed restore" command=restore gl_project_path=snippets/19 relative_path=@snippets/94/00/9400f1b21cb527d7fa3d3eabba93557a18ebe7a2ca4e471cfe5e4c5b4ca7f767.git storage_name=default
time="2021-09-29T14:45:54+13:00" level=info msg="started restore" command=restore gl_project_path=snippets/20 relative_path=@snippets/f5/ca/f5ca38f748a1d6eaf726b8a42fb575c3c71f1864a8143301782de13da2d9202b.git storage_name=default
time="2021-09-29T14:45:54+13:00" level=info msg="completed restore" command=restore gl_project_path=snippets/20 relative_path=@snippets/f5/ca/f5ca38f748a1d6eaf726b8a42fb575c3c71f1864a8143301782de13da2d9202b.git storage_name=default
time="2021-09-29T14:47:00+13:00" level=info msg="completed restore" command=restore gl_project_path=root/gitlab relative_path=@hashed/f5/ca/f5ca38f748a1d6eaf726b8a42fb575c3c71f1864a8143301782de13da2d9202b.git storage_name=default