[PATCH] wt-status: expand, not dwim, a "detached from" ref

When a user checks out the upstream branch of HEAD and then runs "git branch", like this:

  git checkout @{u}
  git branch

an error message is printed:

  fatal: HEAD does not point to a branch

(This error message when running "git branch" persists even after checking out other things - it only stops after checking out a branch.)

This is because "git branch" attempts to DWIM "@{u}" when determining the "HEAD detached" message, but that doesn't work because HEAD no longer points to a branch.

Therefore, when calculating the status of a worktree, only expand such a string, not DWIM it. Such strings would not match a ref, and "git branch" would report "HEAD detached at <hash>" instead.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
I tried to track down other uses of wt_status_get_detached_from()
(called from wt_status_get_state() with get_detached_from=1) but there
seemed to be quite a few, so I stopped.

One alternative is to plumb down a flag from dwim_ref() to
interpret_branch_mark() that indicates that (1) don't read HEAD when
processing, and (2) when encountering a ref of the form "<optional
ref>@{u}", don't die when the optional ref doesn't exist, is not a
branch, or does not have an upstream. (We cannot change the
functionality outright because other parts of Git rely on such a message
being printed.) But this seems too complicated just for diagnosis.
---
 t/t3203-branch-output.sh | 10 ++++++++++
 wt-status.c              |  2 +-
 2 files changed, 11 insertions(+), 1 deletion(-)