Create a flag to build the changelog from commits in multiple git repositories #790

Description

Reviewed #365 prior to writing this and I believe this is different enough to warrant a different issue

Problem
I have 2 git repositories core_repo and plugins_repo. The 2 are developed and maintained by the same team and are deployed together; therefore we would like to have only a single changelog that tracks the changes from the 2 repos.

Possible Solution

Is it possible to add a flag to fetch commits from multiple repos as though all the commits came from a single repository?
I would like to be able to do something like cz changelog --unreleased --repository $CORE_REPO_DIR $PLUGINS_REPO_DIR and end up with a single changelog file as though the commits from both repos came from a single repo.

Additional context

Here is an example to demonstrate this practically
Within repo1

git log
feat: feature 1 in repo 1
fix: bugfix 1 in repo 1
feat: feature 2 in repo 1
fix: bugfix 2 in repo 1

Within repo2

git log
feat: feature 1 in repo 2
fix: bugfix 1 in repo 2
feat: feature 2 in repo 2
fix: bugfix 2 in repo 2

I would like to be able to run cz changelog --unreleased --repository $REPO1_DIR $REPO2_DIR and get:

# Changelog

All notable changes to this project will be documented in this file.

## [1.0.1] - 2023-07-14

### Bug Fixes

- Bugfix 1 in repo 1
- Bugfix 2 in repo 1
- Bugfix 1 in repo 2
- Bugfix 2 in repo 2

### Features

- Feature 1 in repo 1
- Feature 2 in repo 1
- Feature 1 in repo 2
- Feature 2 in repo 2

I am willing to submit a PR to do this but I will need some guidance and a bit of time