path.c: refactor and add worktree_git_path()

path.c: refactor and add worktree_git_path()

do_git_path(), which is the common code for all git_path* functions, is
modified to take a worktree struct and can produce paths for any
worktree.

worktree_git_path() is the first function that makes use of this. It can
be used to write code that can examine any worktree. For example,
wt_status_get_state() will be converted using this to take
am/rebase/... state of any worktree.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>