`magit-ignore-item': make LOCAL arg optional

Commit 15c08f9e merged `magit-ignore-item' and `magit--ignore-item', and
for some reason made the LOCAL arg non-optional, causing Emacs to signal
`wrong-number-of-arguments' when running `magit-ignore-item' ("i") on a
file in the status buffer.

Fixes #893.

Signed-off-by: Pieter Praet <pieter@praet.org>