Add previous commit as context to commit command

[gfargo]

Describe the request
In some cases, it would be helpful to provide the previous commit, or even all previous commits in the current branch, as the context within the commit command.

One example being you've created a bunch of helper functions and type changes in earlier commits and now you're refactoring code to use the new helpers or types... If this was provided as context to the LLM it would be able to infer better how the newest commit is unique in relation to all the ones before it.