branch.h

Add a new function, find_shared_symref, which contains the heart of
die_if_checked_out, but works for any symref, not just HEAD.  Refactor
die_if_checked_out to use the same infrastructure as
find_shared_symref.

Soon, we will use find_shared_symref to protect notes merges in
worktrees.

Signed-off-by: David Turner <dturner@twopensource.com>
Reviewed-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>