Closes gitaly#921

Closes gitaly#929

Closes gitaly#915

Closes gitaly#907

Closes gitaly#851

Closes gitaly#863

Closes gitaly#866

Closes gitaly#903

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

Closes gitaly#825

Uses `list_commits_by_oid` on the CommitService, to request the needed
commits for pipelines. These commits are needed to display the user that
created the commit and the commit title.

This includes fixes for tests failing that depended on the commit
being `nil`. However, now these are batch loaded, this doesn't happen
anymore and the commits are an instance of BatchLoader.

Closes gitaly#808

- Added tests for Git::Commit.shas_with_signatures

Closes gitaly#780

Closes gitaly#737

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.