t5516: test updateInstead with worktree and unborn bare HEAD

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>