Collection of _ideas_ for REST API v3

[snazy]

There is no plan to introduce a REST API v3 yet, but eventually there will be a need for it. This issue is meant to collect ideas in the summary.

• Remove expectedHash from merge-operation, if no longer needed. See this discussion
• Return source-ref-name + hash in merge-response
• Return more information conflicts in merge-response (keys + contents on merge-base, merge-source, merge-target)
• More flexible, commit-to-branch like, way to add changes to the squashed merge-commit
• Rename API parameters called hash + hashOnRef to "commitSpec`, so that it is clear that it does not need to be a commit-ID but can also use a relative spec.
• Add renameTo to Conflict response object