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

hghave: add test for clang 6 and later clang 6 includes integrated libfuzzer support, which makes fuzzer stuff a little easier to work with. Differential Revision: https://phab.mercurial-scm.org/D1873