In basic mode, prohibit commits to default merge branch

[isc-pbarton]

If we are in basic mode and have a default merge branch configured (call it "development"), there should never be commits from git-source-control directly into development. A couple of scenarios where this could happen and lead to issues:

• I switched back to the development branch after finishing a feature and started working on a new feature but forgot to create a new feature branch
• I tried to create a new feature branch but it failed for some reason. I assume I'm on the new branch because I didn't check source control output, but I'm actually on development.

If the current branch is development, the sync option should not add, commit, or push. It should just pull.

[isc-tleavitt]

This ends up being pretty bad because branches can end up diverging and the situation is impossible to resolve without e.g. running classmethods. Added to Q3/Q4 milestone.