t5516: test updateInstead with worktree and unborn bare HEAD

This is a regression test which should presently fail, to demonstrate
the behavior I encountered that looks like a bug.

When a bare repository has a worktree checked out on a separate branch,
receive.denyCurrentBranch=updateInstead should allow a push to that
branch and update the linked worktree, as long as the linked worktree is
clean.

But, if the bare repository's own HEAD is repointed to an unborn branch,
the push is rejected with "Working directory has staged changes", even
though the linked worktree itself is clean.

This test is essentially a minimal working example of what I encountered
while actually using Git; it might not be the optimal way to demonstrate
the underlying bug. I suspect builtin/receive-pack.c is using the bare
repository's HEAD even when comparing it to the worktree's index.

Signed-off-by: Runxi Yu <me@runxiyu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: runxiyu

t/t5516-fetch-push.sh

@@ -1816,6 +1816,24 @@ test_expect_success 'denyCurrentBranch and bare repository worktrees' '
 	test_must_fail git push --delete bare.git wt
 '
 
+# NEEDSWORK: updateInstead unexpectedly fails when bare HEAD points to unborn
+# branch (or probably any ref that differs from the target worktree) despite
+# the target worktree being clean. This seems to be because receive-pack.c
+# diffs the target worktree index against the bare repository HEAD.
+test_expect_failure 'updateInstead with bare repository worktree and unborn bare HEAD' '
+	test_when_finished "rm -fr bare.git cloned" &&
+	git clone --bare . bare.git &&
+	git -C bare.git worktree add wt &&
+	git -C bare.git config receive.denyCurrentBranch updateInstead &&
+	git -C bare.git symbolic-ref HEAD refs/heads/unborn &&
+	test_must_fail git -C bare.git rev-parse -q --verify HEAD^{commit} &&
+	git clone . cloned &&
+	test_commit -C cloned mozzarella &&
+	git -C cloned push ../bare.git HEAD:wt &&
+	test_path_exists bare.git/wt/mozzarella.t &&
+	test "$(git -C cloned rev-parse HEAD)" = "$(git -C bare.git/wt rev-parse HEAD)"
+'
+
 test_expect_success 'refuse fetch to current branch of worktree' '
 	test_when_finished "git worktree remove --force wt && git branch -D wt" &&
 	git worktree add wt &&