Add binding for GIT_OPT_SET_SSL_CERT_LOCATIONS

[aharbis]

Adds a binding for git_libgit2_opts key GIT_OPT_SET_SSL_CERT_LOCATIONS in opts module.

#995

[ehuss]

Thanks! I was wondering, instead of surfacing the kinda weird low-level API (which looks to be inherited from OpenSSL's SSL_CTX_load_verify_locations) if it could have something that is a little more explicit that doesn't require the use of Option. Perhaps something like separate functions:

pub unsafe fn set_ssl_cert_file<P: IntoCString>(file: P) -> Result<(), Error>
pub unsafe fn set_ssl_cert_dir<P: IntoCString>(path: P) -> Result<(), Error>

That way it is a little clearer and more deliberate as to what the caller is doing. WDYT?

[aharbis]

@ehuss Good idea, I've updated the PR with the proposed change.

[ehuss]

Thanks!