Closes gitaly#921

Closes gitaly#929

Closes #42249

Closes gitaly#915

Closes gitaly#907

Closes gitaly#851

Closes gitaly#863

Closes gitaly#866

Migration is done through a small refactoring, which makes us call
endpoins which are performing the same actions for namespaces.
Tests are added to ensure only the project is removed that should be
removed.

Closes gitlab-org/gitaly#873

Due to the refactoring in !16082, `Blob#batch` no longer falls back
to a default `blob_size_limit`. Since `Repository#batch_blobs` was using
a default `nil` value, this would cause issues in the `Blob#find_by_rugged`
method.

This fix here is to be consistent and use a non-nil default value in
`Repository#batch_blobs`.

The problem was masked in development and tests because Gitaly is always
enabled by default for all features.

Closes #41735

When a project uses fast-forward merging strategy user has
to rebase MRs to target branch before it can be merged.
Now user can do rebase in UI by clicking 'Rebase' button
instead of doing rebase locally.

This feature was already present in EE, this is only backport
of the feature to CE. Couple of changes:
* removed rebase license check
* renamed migration (changed timestamp)

Closes #40301

The previous implementation iterated across the entire patch set
to determine the number of lines added, deleted, and changed. Rugged
has a native method `Rugged::Diff#stat` that does this already,
which appears to be a little faster and require less RAM than doing
this ourselves.

Improves performance in #41524

- Add a Gitlab::Git::Conflict::Resolution class to encapsulate
resolution data
- Simplify conflict file collection assembly

Closes gitaly#825

Given the priorities shifted for the Gitaly team, this endpoint does not
get a dedicated endpoint yet. To make it work in a cloud native
environment the request needs to go to Gitaly, not rugged. This is
achieved by rerouting to the generic TreeEntry endpoint.