fuzz: reorganise the path for existing oss-fuzz fuzzers

In order to provide a better organisation for oss-fuzz fuzzers and to avoid top-level clustters in the git repository when more fuzzers are introduced, move the existing fuzzer-related sources to their own oss-fuzz/ hierarchy. Grouping the fuzzers into their own directory, separate their application on fuzz-testing from the core functionalities of the git code, prvides better and tidier structure the oss-fuzz fuzzing library to manage, locate, build and execute those fuzzers for fuzz-testing purposes in future development. Signed-off-by: Arthur Chan <arthur.chan@adalogics.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>