revert: Hide '-r' option in default usage

revert: Hide '-r' option in default usage

The '-r' command-line option is a no-op provided only for backward
compatiblity since abd6970 (cherry-pick: make -r the default, 2006-10-05),
and somehow ended up surviving across reimplementation in C at 9509af6
(Make git-revert & git-cherry-pick a builtin, 2007-03-01) and another
rewrite of the command line parser at f810379 (Make builtin-revert.c use
parse_options, 2007-10-07).  We should have stopped advertising the option
long time ago.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>