patch: implement --dry-run

patch: implement --dry-run

function                                             old     new   delta
static.patch_longopts                                  -     137    +137
patch_main                                          2053    2135     +82
fail_hunk                                            132     139      +7
finish_oldfile                                       119     124      +5
packed_usage                                       32807   32787     -20
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 3/1 up/down: 231/-20)           Total: 211 bytes

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