Function to generate current remote branch

[ruifm]

#22 made me realize that more kinds of useful URLs can be generated.

Now that file (+lines), and repo homepage are implemented, the next one that comes to mind is to generate a URL that points to the current remote branch.

E.g.

https://github.com/neovim/neovim/tree/release-0.5

will always point to the head of the release-0.5 branch, not any specific revision.

[fantasygiveup]

@ruifm Looking forward for this improvement, it makes a link more readable!