diff: disable debug code; remove unsupported --dry-run from help text

diff: disable debug code; remove unsupported --dry-run from help text

function                                             old     new   delta
packed_usage                                       28623   28602     -21
do_line                                              121      91     -30
patch_main                                          2086    1923    -163
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-214)           Total: -214 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>