Branch support

The `gin ls` command assumes the active branch is `master` for a few operations/comparisons, especially when trying to figure out if a file differs from the server/remote. Officially branches aren't supported in the client: Since it can't create branches and we discourage mixing gin and git commands, we consider it an unsupported workflow.

We might support creating branches in the future, but as a first step, dropping the `master` branch assumption and checking the default upstream branch would allow users to use the gin client with branches.