merge-ours: do not use cmd_*() as a subroutine

merge-ours: do not use cmd_*() as a subroutine

The call to cmd_diff_index() "git merge-ours" makes has been working
by accident that the function did not call exit(3), and the caller
exited almost immediately after making a call, but it sets a bad
precedent for people to cut and paste.

For finding out if the index exactly matches the HEAD (or a given
tree-ish), there is index_differs_from() which is exactly written
for that purpose.

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