[Backend] Add RepositorySyncService

[zergov]

Closes: https://github.com/visevol/GihubVisualisation/issues/10

Add the RepositorySyncService class

This class takes a Repository model, and sync its content in our database:

1. It clones the repository in a tmp directory
2. It calls git log on it, and saves the output in a temporary file.
3. The service read the log files and creates Commit and CommitFileChange records for each of the commits.