bdiff: avoid pointer arithmetic on void*
MSVC 2008 complains:
mercurial/cext/bdiff.c(106) : error C2036: 'void *' : unknown size
mercurial/cext/bdiff.c(107) : error C2036: 'void *' : unknown size
Maybe it's a gcc extension?
https://stackoverflow.com/questions/
37460579/error-c2036-void-unknown-size
fuzz: add a quick README to try and document how to test new fuzzers
Differential Revision: https://phab.mercurial-scm.org/D2633
fuzz: add a fuzzer for xdiff
Based entirely on the fuzzer for bdiff.
Differential Revision: https://phab.mercurial-scm.org/D2632