Branch support

[achilleas-k]

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.

[jcolomb]

also linked to using "main" instead of "master".

• gin get sometimes create a master branch if there is none. (and if there is no remotes/origin/HEAD?)