Prevent git push when LFS objects are missing

What

Finds new blobs during git-receive-pack and checks for any LFS pointers. If any of these are not in the lfs_objects table from a prior git lfs push then an error is raised to abort the push.

Why

Prevent repos getting to a broken state where they can' be cloned or have changes pulled.

Performance

• This involves checking each new blob to see if it is an LFS pointer.
• This uses git rev-list to find new blobs and checks
• Fetching blob details is currently done with Rugged when we'd preferably use gitaly
• For repositories which don't use LFS we'll want a way of bypassing this slow check

Todo

• Do we need a way to bypass this if git lfs push --all fails to work?
• Consider having some kind of limit on this for performance
• Enquire about performance impact of current approach

Screenshot

Acceptance criteria

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug

Related

• Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/24564
• Accidentally committed to EE in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2761 which this replaces