contrib: add some basic scaffolding for some fuzz test targets I'd like to get the majority of our C code covered by automated fuzz testing. I've started with bdiff because it was already decoupled from libpython and therefore was fairly quick to produce a working fuzzer. The code here is a little odd because I've been having trouble convincing libfuzzer to define a main and I threw in the towel. This code will also work with github.com/google/oss-fuzz, and once it lands in our main repo I intend to enable automated fuzzing in oss-fuzz with reports going to our security alias. Differential Revision: https://phab.mercurial-scm.org/D1875

formatting: enforce clang-format on all .cc files as well as .c files As part of my upcoming fuzzer patch we're going to grow some .cc files. They're not part of the release build (they're only used for fuzzing, which requires clang 6 which in turn implies a working C++ compiler), so I'm not terribly worried about growing some C++ files. Differential Revision: https://phab.mercurial-scm.org/D1874