worktree: remove extra members from struct add_opts

worktree: remove extra members from struct add_opts

There are two members of 'struct add_opts', which are only used inside
the 'add()' function, but being part of 'struct add_opts' they are
needlessly also passed to the 'add_worktree' function.

Make them local to the 'add()' function to make it clearer where they
are used.

Signed-off-by: Thomas Gummerer <redacted>
Reviewed-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>