refs: add a new function set_worktree_head_symref
Add a new function set_worktree_head_symref, to update HEAD symref for
the specified worktree.
To update HEAD of a linked working tree,
create_symref("worktrees/$work_tree/HEAD", "refs/heads/$branch", msg)
could be used. However when it comes to updating HEAD of the main
working tree, it is unusable because it uses $GIT_DIR for
worktree-specific symrefs (HEAD).
The new function takes git_dir (real directory) as an argument, and
updates HEAD of the working tree. This function will be used when
renaming a branch.
Signed-off-by: Kazuki Yamaguchi <k@rhe.jp>
Acked-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>