Add option to always do branch reviews rather than single-commit reviews from the post-commit hook

[wesm]

Some users may want roborev to always consider the whole branch rather than the default incremental commit review approach, this will give them that option.

This may require changes to the post-commit hook (changing to something like roborev post-commit so it picks up whatever the config.toml-indicated logic should be)

[mariusvniekerk]

we should read this from a repo level config value, and fall back to the current behavior