Summary

This PR implements a function to check if the current branch exists on the sourcegraph instance

If the user is currently on a branch that does not exist remotely, the links-commands will open the repo using the "HEAD" (aka main) branch unless a default branch has been set

Expected Behavior

For local files, users can choose between open / copy the main branch and the current branch.

If the current branch does not exist remotely, it will fall back to the main branch.
If the default branch is set, it will replace the current branch option when clicked.