Add "auto" option for create base branch

[alexr00]

#5195 (comment)

When the repo is a fork, it makes more sense to make a PR against the upstream repo instead of the branch that the current branch was branched off from.

[alexr00]

To verify:

• Make sure that githubPullRequests.createDefaultBaseBranch is set to auto
• Open a folder that contains a clone of a forked GitHub repository
• Check out the main branch of the repo
• Create a new branch
• Run the command "Create Pull Request" (you won't actually need to create the PR, just run the command)
• Verify that the base branch for the PR is set to the upstream repo's main branch, not the fork's main branch.

[lszomoru]

@alexr00, the behaviour works as expected but auto does not seem to be a valid value for the setting.

[alexr00]

@lszomoru I just released a new version of GHPR a few minutes ago. You will need to very latest to test with.