worktree: introduce "add" command
The plan is to relocate "git checkout --to" functionality to "git
worktree add". As a first step, introduce a bare-bones git-worktree
"add" command along with documentation. At this stage, "git worktree
add" merely invokes "git checkout --to" behind the scenes, but an
upcoming patch will move the actual functionality
(checkout.c:prepare_linked_checkout() and its helpers) to worktree.c.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>