Streamline Git credential caching

[marvinkruse]

Problem:
When users are proposing a commit, an error during this process will cause severe problems. This can mainly happen when the users fails to authenticate with their git provider (e.g. GitHub). A typo in the credentials can also cause this.

Solution:
Interact with the git provider (i.e. though a git pull) before starting the proposal oaf the commit and enable credential caching locally. This will prevent these issues.

[marvinkruse]

Client will now execute a git fetch before a commit or finalize (as a proposer) will start, in order to make sure that git won't fail later on.