libgit2: Let fuzzers user internal headers

[nelhage]

Ideally we should
(a) move build.sh into libgit2's own repository
(b) figure out how to use our cmake build in oss-fuzz

but I ran into a bunch of yak shaves with (b), since oss-fuzz needs us to link with $CXX and to pass -lFuzzingEngine. In the meanwhile, this fixes the build.

Alternately, maybe we should export git_config_backend_from_string?

cc @pks-t for your take.

[pks-t]

On Sun, Oct 14, 2018 at 04:58:37PM -0700, Nelson Elhage wrote: Ideally we should (a) move `build.sh` into libgit2's own repository (b) figure out how to use our `cmake` build in oss-fuzz but I ran into a bunch of yak shaves with (b), since oss-fuzz needs us to link with `$CXX` and to pass `-lFuzzingEngine`. In the meanwhile, this fixes the build.

I don't quite care about (a), but I wouldn't be opposed to it. (b) would be a nice thing to have, though, if it doesn't bring too many special cases into our build instructions.

Alternately, maybe we should export `git_config_backend_from_string`?

Yes, I do want to do this in the long run. For now I wanted to keep the API private to be able to iterate upon its signature if required, but I'll probably make this public soon. But either way, we'll need to include private headers in both cases. I've got another PR open in libgit2 right now which makes use of of the private function `git_object__from_raw`.

cc @pks-t for your take.

The current approach looks perfectly fine to me for now. Thanks for handling it!