WIP: directly use --left-right

Modify gitaly client's commit diff call to utilize gitaly's --left-right support

In gitaly#888 (closed), we added support for --left-right in gitaly. This change takes advantage of that added support

What are the relevant issue numbers?

• gitaly#888 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer