Show usage string for 'git merge-ours -h'

Show usage string for 'git merge-ours -h'

This change is strictly about 'git merge-ours -h' without
any other options and arguments.

This change cannot break compatibility since merge drivers are
always passed '--', among other arguments.

Any usage string for this command is a lie, since it ignored its
arguments until now.  Still, it makes sense to let the user know
the expected usage when asked.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>