Add a Remote::default_branch() function

[Sebastian Thiel (Byron)]

Summary 💡

Similar to the libgit2 Remote::default_branch() function.

I believe under the hood this does something like:

git symbolic-ref --short refs/remotes/${remote_name}/HEAD

Motivation 🔦

It would be nice to have an easy way to get the default branch for a remote (if known).

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Originally proposed by Gibson Fahnestock (@gibfahn) in issue #755.