implementation/design: use upstream instead of remote name

[tmastny]

Right now browse uses the remote name to build the GitHub link. It defaults to "origin" which is the typical, default name. browse also has remote name as a parameter and allows users to set a global default.

However, would it be better to use the current branch upstream? This would be consistent with git's interpretation of a "default" remote.

Key details:

git2r::repository_head() %>% git2r::branch_get_upstream()