worktree.c: add find_worktree()

worktree.c: add find_worktree()

So far we haven't needed to identify an existing worktree from command
line. Future commands such as lock or move will need it. The current
implementation identifies worktrees by path (*). In future, the function
could learn to identify by $(basename $path) or tags...

(*) We could probably go cheaper with comparing inode number (and
probably more reliable than paths when unicode enters the game). But not
all systems have good inode that so let's stick to something simple for
now.

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