Default base branch when creating a new branch or PR

[juliojgd]

With gh CLI's gh pr create command the PR is created taking as base branch the current active branch in the repository if it's not explicitly specified. This is the behavior of git client's "git checkout -b new_branch" as it creates the new branch taking as base the currently active branch.

With Sherpa's gh sherpa create-branch or gh sherpa create-pr the branch or PR is always created taking as base branch the default branch of the repository if not specified in an explicit way.

This has led in the past (and keeps happening) to errors where you want to create your branch or PR from the current active branch (that is not the default branch) but Sherpa silently creates it from the default branch.

In order to not be breaking I think a new Sherpa configuration property (in the configuration file sherpa-config.yml) like: "use_active_branch_as_default_base: true" could do the job.
If the property is enabled it means that it will take the current active branch as the base for branch or PR creation, in the same behavior of GH CLI with gh pr create command.

I think this will avoid recurrent errors when creating branches and PR's. In our team this error leads to branch deletion and re-creation several times a week. An alternative implementation would be to require always to specify the base branch in the command line.