Allow incomplete commit records to load their full data from gitaly.

Commits can be based on a Hash of data retrieved from PostgreSQL, and
this data can be intentionally incomplete in order to save space.

A new method #gitaly? has been added to Gitlab::Git::Commit, which
returns true if the underlying data source of the Commit is a
Gitaly::GitCommit.

CommitCollection now has a method #enrich which replaces non-gitaly
commits in place with commits from gitaly.

CommitCollection#without_merge_commits has been updated to call this
method, as in order to determine a merge commit we need to have parent
data.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/58805