Fixes #4287

Briefly, gh pr checkout <pr> [--branch LOCAL_BRANCH] works in two steps:

1. fetch the head branch of <pr> by running git fetch REMOTE +refs/heads/BRANCH:refs/remotes/REMOTE/BRANCH
2. checkout the fetched branch by running git checkout -b LOCAL_BRANCH --track REMOTE/BRANCH

What #4287 reported is that when the fetch refs/remotes/REMOTE/BRANCH is not covered in remote.REMOTE.fetch git config, then the second step failed with

fatal: cannot set up tracking information; starting point 'REMOTE/BRANCH' is not a branch
failed to run git: exit status 128

This PR solves this problem by running¹

git config --add remote.REMOTE.fetch +refs/heads/BRANCH:refs/remotes/REMOTE/BRANCH

if local branch REMOTE/BRANCH doesn't exist after the first step.

Note that remote.<name>.fetch is a multi-valued Git config, though not clearly documented.

Note also that an old syntax git config --add <name> <value> is used to gain compatibility with old Git versions. Since Git 2.26.0, git config learnt some new subcommands and options, see the section Deprecated modes in git config doc.

TODO

• add some code comment
• add new test; Need help mocking a restricted remote.origin.fetch config in test.