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

[Jonathan Tan <jonathantanmy@google.com>]

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(-)

diff --git a/t/t3203-branch-output.sh b/t/t3203-branch-output.sh
index 71818b90f0..c5d3d739e5 100755
--- a/t/t3203-branch-output.sh
+++ b/t/t3203-branch-output.sh
@@ -209,6 +209,16 @@ EOF
 	test_i18ncmp expect actual
 '
 
+test_expect_success 'git branch shows detached HEAD properly after checking out upstream branch' '
+	git init upstream &&
+	test_commit -C upstream foo &&
+
+	git clone upstream downstream &&
+	git -C downstream checkout @{u} &&
+	git -C downstream branch >actual &&
+	test_i18ngrep "HEAD detached at [0-9a-f]\\+" actual
+'
+
 test_expect_success 'git branch `--sort` option' '
 	cat >expect <<-\EOF &&
 	* (HEAD detached from fromtag)
diff --git a/wt-status.c b/wt-status.c
index 98dfa6f73f..f84ebc3e2c 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -1562,7 +1562,7 @@ static void wt_status_get_detached_from(struct repository *r,
 		return;
 	}
 
-	if (dwim_ref(cb.buf.buf, cb.buf.len, &oid, &ref) == 1 &&
+	if (expand_ref(the_repository, cb.buf.buf, cb.buf.len, &oid, &ref) == 1 &&
 	    /* sha1 is a commit? match without further lookup */
 	    (oideq(&cb.noid, &oid) ||
 	     /* perhaps sha1 is a tag, try to dereference to a commit */
-- 
2.26.2.645.ge9eca65c58-goog