Motivation:
Central Dogma uses jGit to store data. Due to the nature of Git that stores unlimited history,
Central Dogma will eventually get in trouble managing disk usage.
We can handle this by maintaing the primary and secondary Git repository internally.
This works in this way:
1 Commits are pushed to the primary Git repository.
2 If the number of commits exceed the threshold (`minRetentionCommits`), then the secondary Git repository is created.
3 Commits are pushed to the both primary and secondary Git repositories.
4 If the secondary Git repository has the number of commits more than the threshold;
  - The secondary Git repository is promoted to the primary Git repository.
  - The primary Git repository is removed completely.
  - Another secondary Git repository is created.
5 Back to 3.

Modifications:
- TBD

Result:
- Close 575
- TBD

Todo:
- Provide a way to set `minRetentionCommits` and `minRetentionDay` for each repository.
- Support mirroring from CD to external Git.

…itory by the leader