Let users specify a fallback / default branch in the editor-to-Sourcegraph extensions

[Joelkw]

When working on a local branch, users may not be able to map 1:1 onto Sourcegraph (which doesn't have their local branch) so we should let them set a default fallback branch so that the editor-to-Sourcegraph connection works smoothly for them.

Full thread: https://sourcegraph.slack.com/archives/C015JREMBTR/p1615396498016700

[github-actions]

Heads up @Joelkw @felixfbecker - the "team/extensibility" label was applied to this issue.

[tjkandala]

Hey @Joelkw, we've had the ability to set a default branch since sourcegraph/sourcegraph-vscode-DEPRECATED#164, but not the ability to set a fallback branch. The default branch is always used, even if there is potentially a remote branch.

I think customers are satisfied based on this comment, so I wanted to know whether we should pursue the full feature or consider this to be done

[Joelkw]

@tjkandala yep, I agree. Thanks for checking!