status: unify parsing of --untracked= and status.showUntrackedFiles

status: unify parsing of --untracked= and status.showUntrackedFiles

There are two code paths that take a string and parse it to enum
untracked_status_type.  Introduce a helper function and use it.

As these two places handle an error differently, add an additional
invalid value to the enum, and have the caller of the helper handle
the error condition, instead of dying or emitting error message from
the helper.

Signed-off-by: Junio C Hamano <gitster@pobox.com>