worktree: teach `add` to accept --reason <string> with --lock

worktree: teach `add` to accept --reason <string> with --lock

The default reason stored in the lock file, "added with --lock",
is unlikely to be what the user would have given in a separate
`git worktree lock` command. Allowing `--reason` to be specified
along with `--lock` when adding a working tree gives the user control
over the reason for locking without needing a second command.

Signed-off-by: Stephen Manz <smanz@alum.mit.edu>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>