fuzz: reorganise the path for existing oss-fuzz fuzzers

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>