Enhancement request: Detect already-linked PR branches when selecting an issue

[ChanceNCounter]

When selecting "Start working on issue" for an issue that already has a linked PR, I expected the extension to check out the existing PR branch. Instead, it prompts me to create a new branch for the issue.

Entering the name of the existing branch works, and the extension furthermore opens the linked PR. However, it's unclear that this will work until you try it, because the prompt uses the usual git branch VSCode prompt: "Enter the name for the new branch."