There's multiple headers included in our fuzzers that aren't required at
all. Furthermore, some of them are not available on Win32, causing
builds to fail. Remove them to fix this.

The `mkdtemp` function is not available on Windows, so our download_refs
fuzzer will fail to compile on Windows. Provide an alternative
implementation to fix it.

By default, libgit2 allows up to 2^32 objects when downloading a
packfile from a remote. For each of these objects, libgit2 will allocate
up to two small structs, which in total adds up to quite a lot of
memory. As a result, our fuzzers might run out of memory rather quick in
case where they receive as input a packfile with such a huge count of
objects.

Limit the packfile object count to 10M objects. This is sufficiently big
to still work with most largish repos (linux.git has around 6M objects
as of now), but small enough to not cause the fuzzer to OOM.

Convert the "download_refs" fuzzer from C++ to C. Rename the source file
to have it be picked up by our build system.