checkout -f: allow ignoring unmerged paths when checking out of the index

checkout -f: allow ignoring unmerged paths when checking out of the index

Earlier we made "git checkout $pathspec" to atomically refuse
the operation of $pathspec matched any path with unmerged
stages.  This patch allows:

    $ git checkout -f a b c

to ignore, instead of error out on, such unmerged paths.  The
fix to prevent checkout of an unmerged path from random stages
is still there.

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