Bug: git checkout @{u} breaks git branch if the upstream isn't a local branch

Consider a repository with branch master, with upstream origin/master. If one is on master, and runs "git checkout @{u}" to get the upstream, then git branch stops working. The reason is that we end up on a detached HEAD, and git branch searches recent reflog entries to figure out where the HEAD was detached from (wt-status.c: wt_status_get_detached_from). Since @{u} is not a local branch, the checkout is inserted into the reflog as "moving from master to @{u}" rather than "moving from master to origin/master". This is because setup_branch_path (checkout.c) only updates the branch name for local branches, keeping the input name (here, "@{u}") otherwise. So, git branch sees "@{u}", decides that we are detached from "@{u}", tries to lookup "HEAD@{u}" and fails with "fatal: HEAD does not point to a branch"

I assume that either the reflog entry should be amended to print something more informative (moving from master to origin/master, or at least master@{u}, or the SHA...), or the reflog search in wt_status_get_detached_from should do something cleverer with relative refs.

- Leszek