worktree.c: find_worktree() search by path suffix

worktree.c: find_worktree() search by path suffix

This allows the user to do something like "worktree lock foo" or
"worktree lock to/foo" instead of "worktree lock /long/path/to/foo" if
it's unambiguous.

With completion support it could be quite convenient. While this base
name search can be done in the same worktree iteration loop, the code is
split into a separate function for clarity.

Suggested-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>